I'll start with the worst by far and away - Microgaming - Stan James, Ladbrokes, Unibet, Victor Chandler etc.
Reason: For tournaments and sit n gos there's virtually no volume; at least 20% of the players are bots (can tell by same play, no chat, reaction time exactly the same, playing numerous tables at once etc.); few if any add on tournaments; both cash and tournament play seems to turn into an all-in fest making it virtually impossible to play properly; no real bonus/cashback for play; graphics leave a lot to be desired; I can go on and on. Also on pokerscout.com it has the lowest rating of any poker network.