I have only played on a hand full of sites, so you might not want to listen to me
.
Personally I like pokerstars the most. They allow you to input pretty much any pic for your avatar, which's fun. And also helps you recognize people quicker.
The interface is decent, i think it can strain the eyes a bit much if you have many tables open, but other than that it's ok.
Players... I dunno you get lots of everything there.
The customer support is great as well...
In the end I think you should just d/l a bunch of them, and try out their play money tables for a while. When you find the site you like the most, go with that one.
Cheers,
Gutzz