I definitely prefer PokerStars, but I have plans to install other programs as well. PokerStars has the best software, and I know they are also the owners of Full Tilt, so I guess that's worth a shot as well.
I started out with PartyPoker, but that just looks horrible nowadays. I don't think I can even access my account. And, because I have a Bwin-account (being a Real Madrid-fan ;-)), I checked out their poker room as well. Ridiculous. I have heard pretty good things about 888 and carbon as well, maybe I'll take them into consideration 🙂