Best at what?

For a beginner, it's probably best to play a warrior first. You'll die a lot, but not so fast that you won't learn anything. Then play what you're most scared of. Rinse and repeat.

Why a warrior? Warriors are tough, and because they're visible all the time, more experienced players will tend to invite you to rank with them (if they're not killing you or ignoring you, that is).

Another good alternative as a first time player, for similar reasons, is a bard.

To do "really well" with just about any class you have to know where your way around a bit, but as long as you don't get scared out of the game you'll have fun with just about anything.

Shapeshifters are also decent, but they have trouble communicating at times, which may or may not be your style.

I classify anti-paladins somewhat similarly -- they're decent, but they have communication troubles which might make it hard as a new player (not the same as shapeshifters, but anti-paladins aren't very good with trust issues).

Rangers, thieves and assassins are in some respects easier to play than a warrior or a bard, but that's because they can hide from so many other players -- again, this limits their ability to communicate.

Good luck.