Best site: Party. If you don't like Party, then I suggest Party. Biggest table choice (especially off peak) and plenty fishy.

The fishiest site of all is Pacific. Their limit games are amazing and there's usually plenty of choice at peak times (not that you need it). Unfortunately using the software will cause a brain hemorrhage, and you can only play one table at a time.

If you're just playing lower limits, Stars and UB are on par with Party although not quite as easy. Don't know about Paradise. Note this is for the lower limits, where I assume you'll be starting.