Pokerstars is number one poker site, with almost instant withdrawals, and a lot of players online.
Also i can suggest you to try ipoker(playtech), best rooms is william hill and betfair, both withdraw within few hours, but there is less players.
To all members, take my apologies for posting invalid links, made mistake, can happen with anyone Thanks admin Valdes for editing, soon i will edit too, and add game names and numbers, so will be great...