Enchant Weapon - Executioner
Some will swear by this enchant, and there's a lot of math involved between figuring out the best weapon enchant for a Retribution Paladin. Certainly, an 840 Armor Penetration bonus is a substantial increase in DPS -- as even bosses have armor to mitigate damage. It also procs often enough that it refreshes, giving a relatively consistent buff throughout encounters. Personally, this is my enchant of choice, stacked with other Armor Penetration bonuses (e.g. Angelista's Revenge). It also arguably has the best animation graphic.
Enchant Weapon - Mongoose
Other players will argue that this enchant is the best. The 120 Agility buff translates to a 4.8% crit and the 2% attack speed increase over 15 seconds is great for a class that relies so much on auto-attacks. The argument between Executioner and Mongoose won't be settled here (it hasn't been definitevely settled anywhere, and I'm not about to step into that particular fire), so it's mostly a matter of preference. This enchant also procs often enough that it sometimes refreshes. The animation for this enchant is greenish electricity.
Enchant Weapon - Savagery
The most straightforward weapon enchant for a Retribution Paladin -- 70 Attack Power. No procs or fancy effects. Just plain old Attack Power. Is it worth it? For players only beginning to do Karazhan and lacking in Attack Power, Savagery is a pretty good choice. It provides a consistent increase to baseline DPS. It also has a pretty cool dripping blood effect.
Glyph of Ferocity
This head enchant is available at Revered status with the Cenarion Expedition, easily achieved through quests in Zangarmarsh, Blade's Edge Mountains, and several runs through the Coilfang Reservoir instances. While Attack Power isn't as desirable as Strength for Retribution, what makes this an ideal starter enchant is the +Hit. If you're not yet Hit capped, then this is a better choice than the...
Glyph of the Outcast
If you have enough Hit (about 95 Hit Rating and 3/3 Precision), you can opt to go for this glyph, which confers Strength and Intellect. With Divine Strength, this scales better than the Glyph of Ferocity. It also gives a little bit of Mana -- good enough for one Crusader Strike, in fact.
There isn't much choice here, the only two questions you need to answer are 1) Scryer or Aldor? and 2) are you Exalted? For those who've chosen to side with the Scryers, you can get Greater Inscription of the Blade at Exalted and its lesser cousin, Inscription of the Blade at Honored. Aldor-aligned Retribution Paladins can get Greater Inscription of Vengeance and Inscription of Vengeance at Exalted and Honored reputations respectively. The difference between the two shoulder enchants are minimal and shouldn't determine faction alignment in any way.
Enchant Chest - Exceptional Stats
Considering there aren't very many Enchants for chest pieces, this is the one that gives the most bang for the buck. Because it increases Strength and, ahem, Agility, it contributes directly to Retadin DPS unlike other chest enchants.
Enchant Bracer - Brawn
If you're only starting to do Karazhan, the idea is to pile on as much Strength as you can muster and get your white damage output to a consistently decent level. This gives better returns than Enchant Bracer - Assault, specially if you have Divine Strength and even buffed with Blessing of Kings.
Enchant Bracer - Stats
A matter of preference entirely, this enchant gives a well-rounded bonus to a Retadin, but doesn't contribute as directly to DPS. Furthermore, the points given to Spirit and to a lesser degree, Stamina, are a bit of a waste. For raiding, in particular, Retadins are better off with Enchant Bracer - Brawn.
Nethercobra / Cobrahide Leg Armor
The only restriction here is purely budgetary. If you can afford the Nethercobra Leg Armor, it's the best enchant you can apply to your leg piece. The 12 Critical Strike Rating is a minor .54% crit increase, but the 50 Attack Power is a considerable boost to DPS. The Nethercleft and Clefthide versions simply don't contribute as much to raid DPS, although the Stamina is attractive for PvP.
Enchant Boots - Dexterity
Since there aren't any Strength enchants to boots, this is the next best thing as it contributes roughly .48% crit, or .53% with Blessing of Kings. The added Dodge and Armor doesn't hurt, either.
Enchant Boots - Surefooted
The root and snare resistance is somewhat useless in a raid situation, but the +10 Hit is pretty substantial, specially for players still gearing up for Karazhan and may be somewhat lacking in +Hit. This is also the most expensive enchant for for boots as it requires Primal Nether, so be sure to apply it only to boots you'll be using for a while.
Enchant Ring - Stats
Available only to Enchanters, a +4 bonus to all stats on both rings provides a modest boost to raid performance, contributing about 17.6 Attack Power with Divine Strength, about .32% crit, and 120 Mana. Enchant Ring - Stats also scales directly with Blessing of Kings. Enchanters can get the Formula for this enchant at Honored Reputation with Lower City.
Enchant Gloves - Major Strength
There just isn't any argument here. +15 Strength translates to 33 Attack Power with Divine Strength, several notches above Enchant Gloves - Assault.
Enchant Cloak - Greater Agility
There isn't much to choose from when it comes to cloak enchants, and this enchant is the best for Retribution DPS, contributing about .48% crit or .53% with Blessing of Kings. It's pretty much a mirror of the boots enchant and is the only enchant a melee DPS should consider for cloaks.
Bold Living Ruby
If you need to fill a red slot, fill it with Strength. If you can afford a Bold Crimson Spinel, don't hesitate to put it in. For entry-into-Karazhan-level gear, however, these blue gems should be sufficient.
If you need more +Hit, yellow gems are an excellent way to obtain them. Because there aren't too many Enchantments that confer Hit Rating, gems are the most common and stackable way to obtain it.
If you're Hit capped, it's not a bad idea to go for crit if you need to fill a yellow slot. A Smooth Dawnstone confers a modest .36% crit, but you can opt to purchase the epic version, the Smooth Lionseye, for .45% if you have the Gold to burn.
Despite the odd spelling, this gem drops off the bosses in Heroic Mana-Tombs and is no longer Unique-Equipped, so you can farm the instance and conceivably fill all your sockets for free. Not quite as good as pure-color gems, this helps fill out those slots to pursue a Metagem requirement. The more common equivalent are the correctly-spelled Inscribed Noble Topazes and the more expensive Pyrestone version.
Thundering Skyfire Diamond
While there is a good selection of Metagems for melee classes, this one springs up as the most significant increase to DPS for players only gearing up for Karazhan. +240 Haste Rating for 6 seconds is pretty good, despite the rumored cooldown of 40 seconds. With extremely high crit ratings, the Relentless Earthstorm Diamond might be a better choice for Metagem. Other Metagems have PvP bonuses such as Snare & Root resists or resistance to Stuns, making them only half as useful for raiding Karazhan.
Be a responsible raider and bring consumables. If you won't be able to top the DPS meters (it's possible, but you'll need to work hard at it), at least pull your own weight by being well prepared. You should bring food, potions, flasks or elixirs, and even temporary weapon enchants. Every bit of preparation helps, and you'll establish a reputation for being a good raider if you're fully stocked.
Spicy Hot Talbuk
Yum. A Retribution Paladin isn't too interested in the Spirit bonus, but the 20 Hit Rating is an excellent buff for players still rounding out their gear. If you're not yet Hit capped, this should be the food to eat.
If you're Hit capped, go for Strength food, which confers better returns than the more common Attack Power food like Ravager Dogs.
Fel Strength Elixir
Far and away the best elixir you can use as a Retadin, the -10 Stamina penalty is minor compared to the benefit -- 90 Attack Power, which is higher than even any of the Strength elixirs. The recipe is a zone drop from most of the demons in Shadowmoon Valley, so only the more conscientious Alechemists are likely to have it.
Elixir of Major Strength
This is the next best thing to Fel Strength (which ironically doesn't confer any Strength at all) Elixirs, as the 35 Strength translates to about 77 Attack Power with Divine Strength. Even with Blessing of Kings, however, this still doesn't give as much raw AP as the Fel Strength Elixir. On the other hand, this is learned from trainers and only uses Felweed and Mountain Silversage, an old world herb.
Elixir of Major Mageblood
Probably a little too expensive if you're only beginning to do Karazhan, but if you have the Gold to burn, this elixir will help keep a constant Mana regeneration rate to make up for the MP5 you won't have in Retadin gear. It requires Ancient Lichen and Netherbloom to make.
Elixir of Draenic Wisdom
This is a cheaper alternative to the Major Mageblood, requiring only Felweed and Terocone, and gives a modest 30 Intellect and Spirit buff. The 30 Intellect translates to 450 Mana, or almost two Crusader Strikes. Not a bad deal for such a cheap elixir. Even though it doesn't contribute directly to your DPS, don't ignore the benefits of guardian elixirs. It's also good raider practice to have both a Guardian and Battle Elixir in effect.
Flask of Relentless Assault
Don't use flasks for Karazhan. Ok, well, it's really up to you. But since they persist through death, they can end up being cheaper than having Battle and Guardian Elixirs up anyway, specially if you keep dying in your first attempts. Flasks indicate determination (or deep pockets) on the part of the raider, and 120 Attack Power is certainly a huge boost to your DPS. If you want to go for flasks, this is the one you should look at.
Scroll of Strength V
Don't laugh. Scrolls are often ignored and used with abandon, but since there's no class buff that confers Strength, nothing can overwrite a Scroll of Strength (unlike Scrolls of Spirit or Intellect, which are superceded by Divine Spirit or Arcane Intellect). This version of the scroll confers the equivalent of 44 Attack Power with Divine Strength. If you're a Retadin, pull out all the stops and slap on every buff you can get your hands on.
Scroll of Agility V
You can have as many scroll effects active as you want, so getting an additional 20 Agility (.8% crit) won't hurt at all. It will also stack with Grace of Air Totem the same way Scrolls of Strength stack with Strength of Earth. Since scrolls are stat buffs (as opposed to Attack Power or health buffs), they scale rather well with Blessing of Kings.
Insane Strength Potion
Incredibly cheap to make with merely 3 Terocone, these potions can work as (and with) a trinket like the Bloodlust Brooch. It translates to a 264 Attack Power buff with Divine Strength. The Defense rating penalty is negligible since you're DPSing and not worrying about getting attacked -- in fact, taking a bit of damage is good so you can benefit from Spiritual Attunement.
Quite arguably the best potion to take in an encounter, specially when timed with the use of DPS-boosting trinkets and Avenging Wrath. Haste will stack with other haste effects, so it works well with Bloodlust or Heroism. 400 Haste Rating translates to a 25.5% increase to attack speed, a significant boost in DPS.
Fel Mana Potion
Although this will hurt Alliance Paladins who rely on spell damage coefficients for Seal of Command, this potion is arguably better than the regular Mana Potions. Retribution Paladins do have trouble with mana and suffer during longevity fights. This potion will help alleviate that problem.
Adamantite Sharpening Stone / Adamantite Weightstone
Unless you have a Shaman giving you Windfury Totem, buff your weapon with either of these Blacksmiting-crafted consumables. As they are weapon buffs, they won't disappear when you die, making them an excellent investment compared to elixirs.
Retribution Paladins are extremely gear dependent. In order to perform well, Retadins must have the best possible gear for their instance level. It isn't enough to simply have a good weapon or decent armor -- it's important to enchant every possible slot in order to squeeze out the best performance from your Retadin. It might be an expensive endeavor, but it's worth the investment, specially when you see the results once you begin raiding Karazhan. Have fun storming the castle!