Well, it depends. MAME will cover pretty much all of your bases...BUT...MAME is more about accuracy than playability. That means most of your newer games will run accurate to the standup, but in some cases too slow to be playable.
"I've tried a few, ADVmame, mame32, etc, but havent really noticed much difference."
The reason you don't see a difference between ADVmame/mame32/xmame/etc. is the fact that they all share basically the same driver set, and it's usually features that change (OS support, input/output advanced options). Essentially they are re-distributable custom compiles based off of the same MAME source code, hence the similarity.
"I see people talking about Kawaks (sp?) alot, what do some of you gurus use to run newer roms (2d fighters, KI, Metal Slug types, etc, mostly the games Ill be playing in my cabinet) ?"
Kawaks is a CPS1/2 and NeoGeo emulator. The goal for that emulator is more focused on playability, and will make resonable sacrifices in the drivers to ensure faster execution (Even though there is code sharing between emus). There's a number of emulators out there for everything, but I would stick to Kawaks for the CPS1/2 and NeoGeo based games. I can't think of anything recent off the top of my head, fighting game wise, that you would miss out by excluding MAME.