The most common meeting place is NCR's bazzar, but a lot of nasty people hang out there, mostly becasue they already know that you basically HAVE to go through there from time to time.
The Hub's main entrance is also a reasonably common place to run into people, and the police tehre keep the worst of the riff-raff out.
Parts of Boneyard can get populaouce, especially Adytum.
Milposa is a pk'ers paradise, since you have to open a window to use the AWB there and have your back to them, meaning they don't even HAVE to sneak up behind you.
The only truly safe places are Hinckley (no weapons allowed) and the regen points (no violence allowed, not that you have anything at regen anyways).