PokerStars: Haves the best varity of games but no rackback(but if u play enough u can use the FTP as a form of rackback.
It also haves the biggest gurantees.
Best trafic around.
Also if your planing on playing the 1$ sng there rack is horrible(20-25%) but if your planing on playing higher the rack is fine.
Haves good low level ring games,lowest at 2NL.
FullTilt: Is the second biggest,you can get rackback but the low level rack is also terrible(20-25% in 1$ sng,and terrible at 10NL).
Lowest ring is 10NL but not worth playing ring on ftp until 25NL because of the rack.
Usualy haves some good promotions running.
Merge Network:Low trafic but soft play,rack is better at the low levels then at the big sites.
Everest: Good rack,decent trafic,gurantees sometimes have over lay and very soft play.
Other then that just check what sign up bonus are running at each site at a time and go from there.
Also check out this link
https://www.cardschat.com/poker-site-reviews.php