This is another pretty common question, so if you click "Search" and put in US Players you can see a whole bunch of answers that the question has gotten in the past.
Right now I play mostly on
bodog and occasionally at Full Tilt and
poker stars. Poker Stars and Full Tilt are almost TOO damn big, though. And, IMO, bigger doesn't necessarily mean better. I keep them for private games with my buddies on PS and I like the SNGs and KO tourneys at FT.
The only time I play UB is for CC freerolls, and if not for those I would probably for sure get rid of Ultimate Bet. That site seems like a bunch of chimpanzees threw it together. (FWIW, I used to play Absolute Poker occasionally, but not after the cheating scandal. I think you would have to be high to deposit your money with UB or AP after that.)
Carbon I haven't decided about yet, though I could do without all of the geeky graphics--save that crap for the kiddies. I'm gonna keep it around, though I'm reluctant to send them any money. Maybe I can put together a little BR through the CC FR and play it a little more so I can make up my mind.
I liked the old setup of Doyle's Room, and now that it's on Cake I'm going to ditch one of those two along with PO, but I still haven't decided which. I really don't see the need to keep them both since they use the same skin. I've already decided to get rid of Players Only (also on the Cake skin).
That's my .02. Check out that search function to get some more input!