As a rule of thumb, the best is that which is paying the highest and staying the longest

If you ask me, in the past one of such campaigns was that of PrimeDice, and I think many former participants of it will confirm my words. There was no limit whatsoever on the number of posts made (and no language restrictions either). It was not uncommon to see some posters consistently making over 1,000 posts a month in total (though of dubious quality), and it lasted for over a year (if I'm not mistaken). As of now, I'm quite happy with the signature campaign I'm currently enrolled in, and I hope that it will continue for longer than PrimeDice's