It all depends what you're looking for. Are you looking for a site with small numbers of entries, but big payouts? Are you looking for a site that you find competitive, and are rewarded for your success? Are you looking for a site that gives you a huge deposit bonus? Are you looking for a site that has no withdrawal restrictions for your freeroll winnings?
My advice is to try many of the major sites, and find out what you like about them and what you don't like about them. Of all the sites I play at, I enjoy Absolute Poker the most. However, everyone has different tastes, and there may be a site out there that you discover has all or many of the qualities you're looking for.