Having played both, I was really impressed with Path of Exile and not so impressed so far with Diablo 3. I haven't played Torchlight 2 yet so I cannot comment (I just received my beta email).
Path of Exile has far better graphics, a far darker atmosphere, more blood, and much better spell/skill animations. I am not a graphics ho, I still play Ultima Online and other older games, for me it is all about gameplay, but Diablo 3 has 5 year old cartoony graphics and that is a huge disappointment. The game is also not well optimized, I get crappy FPS on 2 nice GPU in crossfire and I have friends who need to turn down setttings to play the game to reduce lag even though they are using decent new computers, again not very acceptable for a game with old cartoony graphics.
Combat in Path of Exile can get a bit repetitive, and the story seems more intriguing in Diablo 3 so I will give it that much.
I don't like how stats are automatically assigned in Diablo 3, Path of Exile got it down I think with their tree system. The skill system in Diablo 3 is also not my favorite, I think they could have done much better.
I did have some fun playing D3 yesterday in some random groups, but I'll be waiting for the Path of Exile open beta coming in June 2012.