My would have to be the Archimedes cannon gun and probably the anti-matter rifle. These devastate the Legion and everyone in my path!
I would have to say the Big Boomer is my favourite weapon in New Vegas. It is by far the strongest, most powerful shot gun in the game and is a gun I constantly found upgrading. Unfortunately the only way to get it is by killing the poor old woman outside her garage, so it is heart breaking to get.
Embrace your inner dark side and enjoy taking her weapon from her! ;] You can't pickpocket it from her?
I always leaned towards the plasma rifle and the 9mm. I think that those guns handled most situations for me. The only time I would pull at all the stop is when I had to defend myself against the legion. Besides that, I would only use those two weapons.
The Gobi campaign sniper rifle and the YCS/186 gauss rifle are awesome. Gobi sniper found in the 100 lockpick box in the snipers nest, has it's own repair kit, high DAM and DPS and allows at least 3-4 shots in VATS. The YCS has a very high DAM and one of the highest DPS of any rifle.
