Hmmm, I always play New Vegas with these mods. And to me, they're practically part of the game now.
1. Project Nevada (Pretty much a go to for a bunch of tweaks and improvements and big changes that make the game even better)
2. EVE (Nice looking particle effects and cleaner energy weapons)
3. NMC's Texture Pack (A better looking game!)
4. MTUI (Adjusts the UI for today's big monitors)
5. The Someguy Series (Seriously, they're great)
6. Willow (The best companion mod I've come across)
7. 4GB Fallout New Vegas (gives NV some more ram to work with for people who play with lots of mods)