I can't really say I know of a great one that has a lot of members, but here's a run-down on the ones I know of.
Killsteal Gaming is probably the worst community i've seen. Reason? Drama fucking drama. I am former staff in that community and it was not even worth my time. It got to the point where I felt like resigning every time I got on the teamspeak. Eventually, I got demoted because some bitch didn't like me and made up a bunch of rumors that I was talking shit about her. No investigation was done because she has tits.
Antimeta Gaming is just like Killsteal, except they have a few respectable staff members and less shitty overall members. I am currently on their memberlist and was even offered a staff position. I declined due to the fact that they have a lot of drama infested kids on their staff team. The community itself isn't that bad(much better than Killsteal) but eventually you'd start to hate it.
Dirtnap Gaming This community is the best of the one's i've mentioned so far. I am on the memberlist in this community, but i've spent little time there. I believe they have much less drama due to the fact that they run a completely different model than the other 2(less staff, more professional). This is the only one so far that I would recommend trying.
Rage Quit Gaming is a community in the works by yours truly. We have 0 drama, 0 shitty members, 0 children in our staff. The catch, is we haven't really started up. We're waiting on a few things, but once we get rolling i'm sure we'll be the best community I know of. If you're looking for a community just starting out to invest time in and help grow, RQG is your place. We currently need help with recruitment and organizing events.
tl;dr
Dirtnap Gaming is a good established community to try
Rage Quit Gaming is a community that hasn't gotten off the ground yet, but is a good community to try if you like to be apart of something just starting out.