Depends: Evil, go with Caesar or House. Good, go with NCR or Independent. I recommend independent.
Actually, you usually should be evil to side with Caear, with NCR Good and with Mr. House Neutral. To have Vegas independent you can have all three sorts of Karma. That's allways usual, but you can be Good to side with Caear and Evil with NCR.
It's your decision. Each faction has its own endings, so It should be fun to experience all endings. I played all sides, Good, Bad, or Neutral.