This is an iconic question on these boards, and you'll get a lot of different responses from people who've made different compositions work successfully at all levels of the game.

However, if you want the optimal group, my belief is still that it will be a tank with 4 of the same dps that can offheal well. This generally means the dps are all either shaman/paladin/druid.

The reason I say this is, if all 4 of the dps are the same class, it is VERY easy to keep their actions in sync. This means fewer keystrokes and less mental effort spent keeping the rotation right, and properly using CDs, etc.

Some people disagree with me, and say that a more diverse team brings a greater number of buffs and abilities, and is therefore, theoretically, more powerful. Perhaps true theoretically, but since we (the people driving) aren't computers, we have to factor in how many different things we can simultaneously keep track of. I'm sure seasoned experts at diverse compositions will tell you that they are very comfortable with their class comps, and I'm sure they are. I still assert that, if they had a simpler rotation, they'd have more time to play more effectively in other ways (like noticing what more of your opponents are doing in pvp, or moving away from environmental effects in pve, that kind of stuff). That type of awareness is, imo, more important than an extra CD here or there.

In some ways, this reminds of the argument whether or not to run addon mods in the game. I have a friend that is a great healer, but he refuses to run any addons. Obviously, it's possible to heal very well at the game without healbot/grid/et al, but I always ask him "how much better would you be if you could heal at the same level with even more free time to do other things?"

So that's my opinion on which is optimal. Having said that, I'll tell you to run whatever sounds the funnest to you. Seriously, it's a game, have fun at it and who cares if it's best or not. Or, if best is how you have fun, then I just told you what I think.

Hope it helps.