Honestly? Your own ass.
I'd agree with NCR, except it's just that. A republic. Nothing will get done and if they win at the Hoover Dam, they'll just keep stretching themselves out until someone (likely the Legion, smacks them down).
I'd agree with the Legion, but I'm not up for slavery. Period. Also, I won't allow my soldiers to go in and massacre a village. If I'm not capable of doing it myself, I'm not playing the game correctly.
Mr. House would be nice, but I don't play second banana to anyone (one of the reasons I preferred Morrowind to Oblivion, I'd rather be Dark Elf Jesus than some nameless, faceless soldier who just happened to be in the cell Captain Picard was escaping through). And sure, it's a sweet gig, but I prefer being at the top. Preferably with an army of robots between me and any jackwagons who want to smack me down.
So, really, Independent is the best option. Everyone gets screwed over, I rake in the bottlecaps. Life is good.