Flavor of the month
If you want to work out a good class for you to PvP on, there are some questions you need to ask yourself about play-style, preferences, expectations, and what you want from your PvP.
One of the first things to assess is what you want from PvP. Particularly right now, although Blizzard's team are forever working to change this, there continue to be what are known as "Flavor of the Month" or FOTM classes, often just referred to as FOTM in the forums. These are basically whatever class is overpowered right now, so right at the start of Mists, before the Stampede bug was fixed, that was beast mastery hunters.
And what do you know? There were suddenly an awful lot of beast mastery hunters in the arena. People like playing classes that are overpowered, that have bugs like the stampede one, that basically, for a short while, allowed players to press a couple of buttons and usually win an arena battle. But the FOTM class choice has its downsides as well as its upsides. The upsides are pretty clear, really, being that you're overpowered and likely to do pretty well for a while.
The downsides are that, firstly, the M at the end of the acronym should be given more weight. The FOTM class's overpowered state is only ever temporary, as much as it may not seem that way to its opponents. Blizzard will fix it before too long, and while some classes remain strong for whole seasons, or even whole expansions (I'm looking at you in Cataclysm, rogues) it can sometimes be the case that the nerf rollercoaster takes too big a dip after being on a high for some time. Rogues in Mists are a fine example of this.
So someone looking to work out a class for PvP should consider whether they're the sort of player who wants a FOTM class, or not. What are key considerations for figuring that out? Well, a FOTM class choice has some impact outside the aforementioned. Firstly, you'll need to either have a lot of alts already, or like rolling and leveling alts. While some classes are consistently strong, others come as a bit of a surprise, such as warriors in Mists.
Also, FOTM players, because they have to be changeable to stick with their model of choice, are not likely to stick to something long enough to learn it in a really in-depth way. Players like those winning the world championships in shanghai, so Venruki, Snutz and Kollektiv, haven't been switching around, following the flavor of the month, they've been a mage, a warlock and a shaman for years.
What's strong right now?
How does a player work out what's strong in PvP right now? Well, there are various options. You could look at the top arena and RBG teams for your, or for any battlegroup, and see whether certain classes or comps appear a lot. There's also a site which shows class distribution in 2200+ rated teams, and in general. If a class is highly represented in 2200+ rated teams -- the left column -- but lower represented in general, there's a good chance they're strong.
It should be noted that what's strong right now might not be what's strong in a month's time. Beast mastery hunters are still up there, for example, despite their changes to bring them in line with other specs, likely because they were so strong in the early part of the season. Warriors have had some nerfs, although more are probably still needed. What's strong right now, unless you want to purely follow the flavor of the month, should not completely inform your choice. And what's more, these sites will always reflect something of a sampling bias. It might be the case that, for example, good shadow priests are really good, really practiced players. But, actually, elemental shaman are ridiculously OP right now, with no really good players playing them. Bear that in mind.
But, if, for example, you happen to have a shaman that you play restoration and elemental on, and a priest that you play holy and shadow on, you can use resources like those linked above to assess which of those specs might be a good choice for you at the moment. However, it can be far less fun to play a spec that isn't your favorite just because it's the one that's strong.
Sticking to your guns
This is rather the opposite approach to a flavor of the month player, I'm presenting you with two extremes, you see. Someone who sticks to their class through ups and downs, through thick and thin, is the opposite to a player who is always switching to follow the latest fad. I have a friend who has played a warrior through thick and thin. He's a really, really good warrior, and has weathered all the recent warrior (blade)storms, and is now riding high on the wave of overpowered-ness.
If you really love a class, and/or a spec, there's a lot to be said for sticking to your guns. Really understanding the ins and outs of your class will make you a better player, in PvP and in PvE. What's more, you'll be playing a class and spec that you love, and there's a lot to be said for the effect that has on your enjoyment of the game.
So you need to ask yourself what kind of player you are. Are you someone who wants some (potentially) short-lived entertainment, who might just ride the rollercoaster of nerfs and buffs to success, or are you someone who wants to hold on to your favorite class through the rough and the smooth? It's hard to write that without implying that one is superior, but there really isn't a right answer here. Both are completely valid choices.
I'd suggest a medium somewhere between the two extremes. Work out a class that you think will suit you, and take the information about the strengths and weaknesses of the specs in PvP at the moment into account. If you really, really, really love playing a combat rogue, and hate arms warriors with a passion, you're probably going to have more fun playing a combat rogue than an arms warrior, regardless of where the two specs feature in the chart.
What class is right for me?
But if you haven't got a spec which you particularly like, what class is right for you? This is a hard question to answer. If all the preceding information hasn't helped you at all, and you're starting from scratch, it's a good idea to think about the classes that you play right now, and what you like and dislike about them. Like caster DPS but not melee? Well, stick with that idea. Struggle with boss positioning in melee? A rogue or a kitty druid probably isn't the best class for you. Enjoy annoying melee players? Frost mage.
Or maybe you like the feeling of running in, wielding a big axe and smashing in the faces of your foes? Melee, clearly. Arms warrior or a retribution paladin would probably be a good choice. Like shutting down casters? Again, a warrior is probably the class for you. All the classes have their strengths and weaknesses, the chinks in their armor or their Achilles heels. Consider the type of damage, too: heavy crazy burst or slow ramp-up? Do you want to play a healer, or maybe with a healer?
In an ideal world, you'd have a go at all of them and see what feels good, and if you've the money to spare and places are available, tournament realms are a really great place to try this out. Do bear in mind, though, that PvPers on tournament realms are often very good PvPers -- there's a higher concentration of serious PvP taking place on them per capita than on normal realms. Don't be disheartened!
And above all, make sure you're having fun. PvP's better if you're enjoying it! What about you, what do you play for PvP, and why?
Do you want to capture flags, invade cities, attack towers, and dominate the enemy for your faction? Do you dream of riding your War Bear with pride? We'll steer you to victory with secrets of Battlegrounds and Arena, prepping you with proven addons and keybindings that win! Send questions or comments to firstname.lastname@example.org.