Shack-Tac seemed pretty cool, I applied when I was younger than 18 and of course never got in. By the time I was 18 the arma 2 community was well on its deathbed.
I was invited to a 15th MEU event once, and got to knew the guys there (Specifically Penny, the 2IC). Not sure if the old guys are still around, but that community was hard core.
They had quality ops. We aren't blessed with that sort of stuff anymore.