I change sites every once in a while based on where I think I can make the most, but currently at Titan I'm getting 35% rakeback, plus bonus (essentially bottomless), plus VIP perks which works out to just over 50% rakeback I think. Combine that with soft players and decent table selection at my level, and it's perfect for me; however, you can do much better than 50%, depending on your game, stakes, etc.
I'm not up on my site research, but take a look through
http://www.bonuswhores.com
http://www.pokerlistings.com
http://forumserver.twoplustwo.com/po...Board=rakeback
Also, consider getting a prop player deal (just Google 'prop poker player'). You usually have to play HU and short-handed, but you can find deals that pay up to 130% rakeback.