This can cause some confusion in what to build, and today we're going to talk about what gives you the most bang for your buck. This article will focus heavily on offense rather than defense for AD champions. We'll cover defense in a later week.
Explosive power gain
Attack damage champions magnify in potency far more than other types of characters. Building Ability Power on a mage gives a linear increase in power, although it can be slightly magnified with Rabadon's Deathcap (which gives a percentage increase to all bonus AP you have). Building tank is slightly multiplicative as well; each point of armor or MR you get is 1% effective durability against that type of damage, and each point of health you gain is actually multiplied by 1+(armor or MR/100) against the appropriate type of damage. Physical attacks are a different story altogether.
Attack speed and its limitations
Attack speed modifiers multiply your champion's base attack rate, which in turn multiplies her DPS. It does not work well for quick hit-and-run harassment, but in any sustained engagement, increased attack speed is a great stat. IAS items add with the bonus IAS you get from leveling; they do not multiply. This means characters with very high base attack speed (like Kennen and Teemo) gain a far greater benefit from IAS items. Attack speed also increases the delivery of on-hit abilities like Kayle's E or Twitch's passive.
Increased attack speed gives the most multiplicative benefit to a champion. IAS is cheaper to raise than critical hit chance (though many items raise both), and it continues to give benefits even past the +100% mark.
Attack speed caps at 2.5 attacks per second, but the items needed to reach this are not ideal choices. It takes more than three Phantom Dancers and a pair of Berserker Greaves for any character to reach the cap, and generally focusing this much on IAS is not favorable compared to building more attack damage. This means that as long as you are buying other non-IAS items such as Infinity Edge, you will not need to worry about hitting the cap. Builds that focus a lot on on-hit effects by using Wit's End, Ionic Spark, or Madred's/Kitae's Bloodrazor have a strong likelihood of wastage, especially on characters with a fast base attack speed like Teemo or Twitch. In general, I would stick to having a maximum of three IAS items, even for on-hit builds.
Attack speed does little for champions who are not constantly attacking. If you are relying a lot on poking or champion abilities, attack speed may not be the best stat to raise. Additionally, attack speed does very little to help you last-hit in the laning phase, so it is a poor stat to raise early. For junglers, IAS is amazing as they rely on raw DPS.
Consider this, however: If an engagement lasts for only a couple of seconds and you manage to get three attacks off instead of two before having to disengage, IAS gave you a 50% increase in damage. In teamfights, IAS gives very, very large benefits. Just make sure you're constantly attacking; if you're not, itemize a different stat.
Armor penetration reduces an opponent's effective armor, making your attacks hit for more damage. All enemies have armor, and that armor increases as the game progresses even without any +armor items.
Armor penetration is the go-to stat for attack damage champions because it is reliable. It gives you a multiplicative damage boost in all situations. It does not rely on random chance, and it always works because enemies always (except in a few very rare situations) have some armor. It doesn't rely on extended exposure to attack multiple times, and it applies to physical damage abilities such as Graves' buckshot or Sivir's boomerang blade. It even applies to physical damage pets like Yorick's ghouls or Malzahar's voidlings.
Flat armor penetration is mostly a squishy-killer. Even against a full load of armor penetration runes, a single Chain Vest will stop a lot of physical damage. Adding a Brutalizer or Youmu's Ghostblade to the mix doesn't make things much better, especially if that Chain Vest becomes a bigger item like a Frozen Heart, Randuin's Omen or Thornmail. The diminished effectiveness of flat armor penetration in these situations doesn't mean that you shouldn't buy Youmu's Ghostblade, but you probably only want to buy it if you want the active ability or the cooldown reduction.
Percentage armor penetration from masteries and Last Whisper are another story. Last Whisper will rip through armor; it can give you nearly double the damage on high armor targets. Even against a 100 armor enemy (fairly average for lategame squishies), Last Whisper will take you from dealing 50% damage to dealing 62.5%, which is about a 20% damage increase. I consider Void Staff (the caster equivalent) to be somewhat optional depending on the enemy build, but because of scaling armor, Last Whisper is almost always a good buy in the lategame.
I randomly take all your health
I love and hate criticals. Crit chance is one of the most troublesome stats in the laning phase of Summoner's Rift because it is so polarizing. A Gangplank with 5% crit chance can get lucky with two crit Parrrleys in a row and force you out of lane. Although there are a lot more reliable ways to win a lane, building Brawler's Gloves as your first item can often lead to free lane victories that you didn't otherwise deserve. Nothing is worse than getting crit two or three times in lane and losing your lane because of it.
Crits are entirely powered by the RNG, and sometimes random numbers don't go your way. I recommend adding in a single crit chance mark to most builds just to get a small chance of random silliness in the early game (1% chance of a free lane win on any early harass is nice), but I don't advise relying on it by buying early Brawler's Gloves or by maxing out crit chance runes.
Part of the reason I don't recommend getting a lot of crit runes is that crit chance maxes out easily. An Infinity Edge and Phantom Dancer give you 55% crit chance with just two items. You could easily grab Youmu's Ghostblade, Atma's Impaler, Cloak and Dagger or any number of other +crit items that also give you other stuff that you want.
Crit is not as good when paired with on-hit effects, as those effects aren't magnified by crits. However, this is not always the case. Some abilities like Gangplank's Parrley and Sivir's Ricochet can crit for full effect, but these are generally rare. Most abilities, such as Shyvana's Twin Bite, can only crit for the base attack component and can't crit for any of the bonus effects.
Even with all that, crit is one of the most important stats simply because Infinity Edge is one of the best items in the game. It gives +50% crit damage and 80 bonus AD, the highest base AD of any item. Improving upon IE by stacking more crit or increasing your volume of crits (by adding IAS) is incredibly powerful.
Building AD is kind of boring. It adds a flat, linear number to your champion's attack damage. AD is a so-so stat to raise if only because champions already have AD as a base statistic and it increases as they level. Even the weakest champions in the game have over 90 AD at level 18 without items. This means that hyper-expensive AD items add quite a bit of damage compared to a champion's base numbers, but there's little reason to raise AD once you've picked up a couple of big ticket items (Infinity Edge and Bloodthirster/Sanguine Blade). After just buying IE, you'll find that crit items actually become much more valuable. A 20% crit item like Cloak and Dagger will give more effective damage per hit than another B.F. Sword.
In general, AD is best early game because it gives more leeway for last-hitting. It's also great on champions with abilities that scale to AD, such as Graves or Caitlyn. AD runes are generally considered very valuable; in the early game, they make last-hitting much easier, and in the later game, runes have a lesser effect.
Putting it all together
First, in the Summoners' Rift laning phase, last-hitting is very important. This makes AD runes and items like Doran's Blade very useful. You can also buy items like Madred's Razors and Wriggle's Lantern to help sustain as well as farm.
Armor penetration is very powerful, especially the 10% mastery and Last Whisper. Percentage penetration scales well into the lategame, when foes have greater armor. In the early game, flat armor penetration runes can be very effective at helping you last-hit or harass your enemy. However, lategame armor penetration runes can fall off a bit compared to crit damage runes in terms of raw performance.
Critical chance is the most important stat for a pure autoattacker, as it provides a massive boost to performance and multiplies strongly on Infinity Edge, which is a must-buy item for most AD champions. It also isn't limited by exposure like attack speed is; even a single poke can score a monstrous crit.
Attack speed is very useful in small amounts to help with last-hitting and getting an extra shot in a fight. It boosts jungling speed greatly and helps with on-hit attacks but is otherwise somewhat weak since most fights involve more moving than shooting or slashing. It also doesn't boost the power of attack-based spells. However, IAS is very cheap compared to most other effects, so squeezing in some IAS is always a good idea. Just don't go too overboard with it, as many champions have attack speed steroids already, and it doesn't really help in fights where you're not constantly autoattacking.
All in all, the standard multi-Doran's opening followed by a sustain item or Infinity Edge is a standard for a reason. From there, you probably want to get some crit or IAS followed by some defensive items. Next week, we'll talk more about itemizing for defense. Until then, good luck and have fun!
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