Cash:
Full Tilt - My favorite. I like the software and since I am winning there, I'm staying. Not the easiest regs but I like the challenge that gives me. And you can always pick a table where those regs aren't playing.
Red Kings - Easy games if you are patient enough but sometimes the opponents are too aggressive for my liking. Don't like the software though. Still, I occasionally play there as well, especially if I'm having a downswing at Full Tilt. It's a plus that you can also pick up "Anonymous" table where you're nickname is hidden as well are others.
Tournaments:
Party Poker - Suitable tournaments for my
bankroll and good tournament selection. Software is fine.
On a side note, start trying out different sites and you'll find the one that suits you the best. I probably tried most of them until I found the best ones for me.