I hear it all the time, what class is the best to solo with. What class is the best to tank with. What class is the best to heal with. What class is the best for DPS.
Want me to let you in on a little secret here?
Any class is the best for anything providing you are good at it.
Sure, some classes and combos are not as ideal for some things such as soloing. I am not about to go soloing heroics on my Disc Priest but I can do it on my Hunter. Outside of that, all classes are basically created equal when it comes to anything that is not soloing.
If you put the time and effort into learning your class and get good at it and I do not just mean a rotation and the such but knowing everything your class can do then you can get the maximum potential out of it.
So whatever the flavor of the month DPS class is I can go and play it and read everything and gem and enchant and gear appropriately and I will still never pull the numbers I do with my Hunter.
Does that mean my hunter is that much better? Absolutely not.
But it does mean that I would be better playing my Hunter then I would be playing a flavor of the month class.
In the end, the best class for you is the one you know the best. Every person has the ability to be good at what they do if they put the effort into it.
I've seen so many ret paladins complaining about how they are messed up bad in PvE now. I would love to see them try and explain why I've had groups with ret paladins doing 15K single target DPS if they are so messed up. That seems like a pretty solid number to me.
The thing is they are just better at playing that class. Sure, if they could apply that same level of skill to a hunter they could perhaps do 18K instead of 15K but there is factor in there that most people that ask that question never consider. How long did it take that person to get that good?
I've played with a few people that are really good at picking up on things and can switch classes and in a week be ruling the game on their new flavor of the month class. Most people need more time then that.
I can do that on my Hunter because I have probably spent more time playing Hunters then any other class in the game, most likely more then every other class combined to be more accurate.
When asking what is easiest to level people love to say hunter and I would be hard pressed not to agree. However, that is because I play a hunter. If you where a first time player I don't actually think a hunter is a very good starting class.
Pet control for one, you will get yourself killed so often from a pet that decides to bring friends if you are not careful. No healing is another bad part of hunters. Hunters are not really a starting class. Easy to level if you know what you are doing? Yes. Easy to level otherwise? Not as much as other classes.
All things being equal, the class you take the time to learn is always going to be the easiest one.
Even the much maligned mage is misunderstood. One button wonder right? Not exactly. Sure you can throw out some decent numbers doing that but does that mean mages are easier then other classes? No way.
There is a big difference from spamming one button and doing 8K DPS, fine for a heroic, and doing the right thing with the same gear and pulling 16K. That one button can get you by for some content, but there are a lot more to do then just that.
So that brings us to the root question, what class is best?
The answer is actually simpler then you would imagine.
The class you like to play is the best class in the game.
If you like playing a class you will always be better at it. If you like playing a class you will make efforts to learn to get better. If you like playing a class you will do anything you can to make that class shine when you play it.
Whatever class you like to play, that is the best class in the game. For you.
If you think warrior tanking is hard it is not because it is hard, it is just because you have not taken the time to learn it. Once you do, it is as easy as any other type of tanking.
Personally I suck at my Rogue. I read the guides, I know the rotation, I know what to do, how to do it, how to gear, enchant, gem, you name it. I just suck.
The reason I suck is not because I am not capable of being good, it is just because I do not like the class. Maybe I could get past that by trying harder but I will never in a million years reach my maximum potential if I do not like the class.
The key to success in any class is enjoying it.
So while you might roll a priest to help your guild out healing you might get good enough to get by but you will never be half as good at it as you are on whatever your main is unless you like doing it.
Enjoying what you are doing is the most important thing.
I do not enjoy Rogues so mine sits at 85 and sometimes I bring him out to join in on a random as a 5th or to do some skinning. He just sits and rots otherwise. Even if something changed in game where Rogues where the #1 over all DPS by a buttload I would not play it. I would never reach those numbers, not because I do not have the skill but because I do not like playing it. Trust me, it shows when you do not like playing a class.
Whenever someone answers a best class question they are answering based on their own personal opinions. There is no real best class.