I can only offer my opinion on Full Tilt, PokerStars and Titan.
FT has the best interface, and generally better players. The bonus cash is fairly easy to clear. The ring games are not soft, I've done fairly well at the MTT's there as you generally don't run into fish playing absolute junk that get lucky. All in all, better players all around. The down side is the MTT's are more expensive. Very few $5 buy in MTT's, although the SNG's have small stakes and are fun.
PokerStars has loads of fish, the interface isn't anywhere near as good as FT either. The plus side is there are lots of micro ring games and very small buy in MTT's. I love playing MTT's, and I'm cheap so I like PS for this fact. The play on PS is the softest of the 3 sites I've been on. Just yesterday I tried a Limit Omaha High/Low SNG just for fun as I've never played that before. Before I knew it there were only 17 people left out of 45? Mind boggling the fish on this site.
Titan has good customer service ( only site I've had to deal with CS, so take it for what it's worth ). The number of people playing is rather small compared to PS and even FT so the SNG's fill up slowly. The play is comparitive to FT, not as many fish as PS by far. The bonus money is as close to impossible to clear as one can get unless you play far bigger stakes than I play. The interface is ok, the SNG's you start out with 1000 chips as compared to 1500 on FT and PS.
Of the 3 I like FullTilt for the level of play and the interface, but I find myself on PokerStars most of the time for the big MTT's that only cost $1 - $3 to enter.