I play quite a bit (for the sign up bonuses, and free stuff) and partypoker.com is definitely the biggest/best room out there!
I've got a little write up on my website on how to get free stuff (casino quality poker chips, amazon.com gift certificates, etc.) by signing up for various casinos!
www.idonk.com/poker.html
If you have any questions let me know and I would be more than happy to help you out!