One thing that bothers me in this game is that you can get the armaments of a large army but at most you fight, like, 6 enemies at once. Even then most of your conflicts are 4 on 1 (or 3 with companions). As a side note, Explosive characters are at a disadvantage as fully perked explosives are devestating and all the large engagements I can think of have either hostages or numerous allies in range.

Anyway, getting to the point, where are the best spots for killing a lot of things that want to kill you back? Only ones I can think of are Camp McCarran (if you're anti-NCR), the Fiend vault, and The Fort. Even the NCRCF barely counts.

If you are on the PC you can use mods that up the spawn count.

I personally feel that at times that there are still too many enemies. Most of the time the density is about right for me. Then again I do not really like FPS games unless it is TES/FO3/NV.

[edit]

As for areas almost any place where Casadores(spelling?) and Deathclaws hang out at can be hectic even if the numbers are low and you do not use VATS.

Also the area where Caesar is at.