For me it is Yobit of coarse !! There can be no other !! It's even in my signature ^^
Well besides them I do like coinexchange and novaexchange.
I think alcurex has potential but right now they simply do not have enough coins. There are many coins I have no idea why they do not have. I have used bittrex and it was just fine no problems at all. I use them as secondary if I find a better price there. I have tried bittler too. ( I think thats the name) I can't remmeber so I guess they were't that great.
Yobit!
There is no support, and constantly people on the troll box seeking support to old tickets.
I would say that makes it the worst. If you can not trust it to answer support questions, some of which will be about lost coins, or be costing the person money, then what have you got but a sham exchange.
Nova is okay in my experience, and may takeover from Yobit, as I sense it is upping its game.