I'd do what fonz suggests. download some of the ones you like the look of and play for about an hour on each on the play money tables to see if you enjoy the layout and the functions on them. I know it sounds silly but there are some sites I can't play on because I don't like the sound they use for the chips and things like that. I play on ultimatebet, empire and partypoker although I spend 90% of my online time at
www.uk-poker.com which is run by victor chandler. I know a lot of people don't like the software they use but it was the first site I played on, I like it, it's got good tournaments, freerolls and easy low / mid stakes no-limit tables