It all depends on what you enjoy. If you're from the U.S. there are a few more choices than just ACR or Black Chip ( 2 of many on the Winning Poker Network but those 2 are the most popular ) because of the existence of Bovada,
intertops ( or
juicy stakes, take your pick ), BetOnline, Dragon Room ( that's right, I said "Dragon Room", a site that 5dimes threw together to compliment their slightly more famous "Shark Tank Poker" room. ), SealsWithClubs, and maybe a few more that I don't know about.
Of the above, I sort of prefer the sites on the Winning Poker Network which include ( besides ACR and Black Chip ) 5dimes ( just the Shark Tank version ), Bookmaker Poker ( which has poker as an afterthought to their sports book ), True Poker ( the original "skin" of the WPN ), and a few others I don't play on.
The reason I prefer the sites on the WPN is personal...I just like the fact that their tourneys are totally skewed awkwardly ( for example, the .55 on demand tourneys have a great structure while the $1.10 and higher ones have a crapshoot structure ), and the fact that you can compete in a crapshoot structure tournament even though you're only an average poker player ( like me ) and have a built in edge because of the incredible stupidity of most of your opponents ( entering with 5, 3, 2, even 1 big blind or less with the hopes of getting incredibly lucky ( which they often do ) ) while you, the player with average intelligence, joins at or near the beginning of the tournament when, during those rare times when the idiots don't suck out on you, you can come out on top.
The reason players on the WPN are allowed to enter tourneys with 5, 3, 2, or even 1 big blind or less is because, driven by greed, the WPN allows outrageously long late registration, which takes into account the fact that most poker players are absolute degenerates that refuse to give up after they bust out so they can't resist clicking on "re-enter" whenever it's allowed...If the average player on the WPN were allowed to re-enter when all they got was 1/10 of a big blind or even 1/100th of a big blind for their hard-earned poker funds, they would do it, even if it takes the rest of their poker
bankroll to do so.