Pokerstars has the most freerolls, but you need a lot of patience. Getting the right place to get a ticket is difficult, you need a lot of luck, because it's mostly play always all-in. Much more interesting tournaments in Natural8. They are also frequent, every hour. There are usually not so many players, the level is not very high. Of course, the prizes are not big, you get $4 for the first place. In summary, both natural8 and pokerstars are good, you just win more often playing Natural8.