For a U.S. player trying to stay "legal" , I am playing ACR and BOL(Bet-on-line), Bet on line actually has picked up some traffic lately, It seems to me anyway. They have alot of freerolls and Guaranteed MTT's. I am still trying to find my "niche". I hate to say it but the rake in the Micros on ACR really affects my winrate so I am becoming more of a MTT, / SnG player. I play $1/2 live poker so I guess I should be playing cash on line but it just doesn't seem as profitable.
For MTT its definetly Pokerstars. For cash money ring tables you can find better sites because of tough competition/sharky pools. However, MTT rewards leaves no doubt that one shall choose pokerstars - for one and the same buy-in amount pokerstars have much larger guaranteed prize pools than any other site.
This question can not be answered unambiguously, as the many components. Play for fun, try to win the number of freerolls and promotions, site interface, a contingent of players and much more.
Test everything. The one that works best will be the best. Each one is better suited to a room type. Good room is one where you can win. Go testing and do not stay in one. Always vary in the rooms, I never got anything on 888poker I started a journey of sit & go 0.10 until I adapt now I am well adapted.