I've played on several different sites and as (bad) luck would have it have gone broke a few times . As a result I've had the opportunity to play and give my personal recommendation for the best online freerolls.

1) Bodog. Not a lot of freerolls but they pay much higher amounts $300-$500, and you are facing 2-3k opponents in most of these.
2) Fulltilt. Only because there are so many freerolls but these usually get upwards of 7-10k entrants.
3) Minted. These are probably the easiest to cash. Not a lot of players so they go pretty quickly and the pay is deep.
4) Carbon. Higher payouts than Minted and a lot more players.

Fulltilt would be the easiest to get into with out a long wait.

What I like most about the Sportsbook poker room is that it gives you the chance to win a free seat in WSOP as they have a special giveaway and it was only open for all freeroll qualifiers, so I must say this is teh best way to have a chance to play in WSOP.