I would say ACR. Reasons being:
1.) I am pretty sure they have one of the largest player pools
2.) The cash games buy ins range very low ( $0.01-0.02 NL), so that is nice for the new/micro stakes players with lower bankrolls. They also have the other stakes covered as well.
3.) There are usually enough other players around so you don't have to be stuck with the same crowd too often.