In a game like Diablo 3 that eventually becomes all about farming up materials and gear the question inevitably arises: Who is the best farmer? The trick to getting a proper answer to the question ends up mired in the details of how classes play and what exactly farming entails. Right now the reward structure of D3 is such that the best way to farm is to find a difficulty level that allows you to mulch mobs effortlessly and only dial it up once you can easily smash the next difficulty level up. There simply isn't any benefit to playing on higher difficulty levels aside from variety and challenge because each difficulty level increases drops of legendary gear about 15% over the previous level but increases monster health and damage by 50%
Clearly if the monsters are threatening your character in any significant way or if they are taking a long time to burn down it is going to be far more efficient to play on an easier difficulty setting. This warps the question of which class is best substantially because the metric is no longer about being able to beat really serious challenges. It is very useful to stop worrying about defenses once you can handily survive in your chosen difficulty and focus solely on blowing up monsters more quickly and moving more rapidly across the map. What all of this boils down to is that ranged classes are very much favoured over melee classes because they can more easily mow down enemies without having to actually walk around very much. Every moment walking towards enemies is a moment not spent blasting them to bits.
My class and spec of choice is a pet focused build at the moment. The great thing about this build is that my pets fill the screen with blockers and tie up enemies and they deal an unbelievable amount of damage. When a boss gets surrounded by my pets they start taking ~4000% damage a second, which is going to be somewhere between 4 and 8 times the sustained damage other people put out. The trouble is that my pets are very stupid and spend a lot of time wandering around, getting caught on terrain features, and being in each other's way. If I was fighting enemies for a very long time and they all had time to get set up and stop being idiots I would be absolutely brutal, no question. The trouble is that when the optimal playing style is to blow up enemies in 1-3 seconds a lot of the time my pets don't even get into position to take a swing before the enemies are dead.
It appears to me that the best possible character for farming is going to be either a wizard or a demon hunter. Both of them are ranged and can melt enemies effectively without running around and they both have great up front damage builds with powerful long range capability. Whether or not these classes continue to hold up if the measurement becomes more focused around beating really hard challenges on the highest difficulty is unclear to me. I know for a certainty that when the enemies hit hard enough and last long enough that kiting becomes necessary that it will be amazing to have super tough blockers to stand around and get punched. It seems likely that as the game gets more difficult the glass cannons with high burst become less and less effective.
This isn't to say that farming capability is necessarily the best measure of a character of course. Given that beating the game is incredibly easy if all you want to do is finish the plot I don't know that there is any real cohesive endgame at all. Much like D2 there is only endless progression, farming to get better and better stuff just to show it off to everyone. Perhaps that is the key - make beating the game fairly straightforward for the casual players and then make a big playground for all the hardcore folks and let them define their own rules.