I asked the same question a while back. It helps if you're really specific about your needs, budget, etc., as CL400 said.
If you want to see what others had to say to me about my question, click here
There are other posts like mine out there, too. Just be specific and this community will spring to action!
My rig is detailed in my sig, but here are some more specifics: GK 700RB-II
: 480 watts at 4ohms (320 @8ohms). Purchased used at GC for $299+tax+$25 warranty. Headroom to spare. Ampeg SVT-15en
: 200 watts @8ohms. Front-ported. No tweeter. F----g crushes ... I got one for $159+tax at GC and the other for $180 on CL. They're somewhat abundant, and there are a few other varities (210he, 410he). Note that the SVT-15en is the same as the SVT-15e.
I love the GK/Ampeg combo (so much, in fact, that I started a club
!). I can get modern or vintage tones, although no one is going to hear me and think, "Ampeg V4B!" And the power is enough to handle pretty much any situation I think I'll find myself in as a musician.