Pokerstars has the most people and the most freerolls, etc...
However, you might as well enter the lottery if you play the freerolls/low-stakes, since you cannot play real poker. Beginners almost never fold and on PStars stay in to the river where they always seem to catch something to beat you
The play becomes normal once you start around the $10 buy-in and above for tournys or sngs.