La fitness, though the one in northridge is garbage. If you are willing to drive, hit up the Santa clarita or Valencia locations. There used to be a gold's near csun, but that closed and Is now some generic fitness center. The only "hardcore" one I can think of is powerhouse in northridge. Never been, but my friend goes and said it's pretty good. Oh, there is also a few 24 hr fitness', northridge having the newest one. And of course, ballys, but they're all garbage.
Personally, I go to la fitness. $30 a month, no contract. I do drive to Santa claret though for the better locations.