If I were me and not the Courier? Mr. House. Legion is out for various reasons (not the least of which is their treatment of women, of which I am one), and the NCR government is corrupt and inefficient. NCR is certainly better than the Legion, and there's always the chance that they could reform and get better, but I wouldn't count on it. House may be a bit on the side of ruthless and calculating, but for the most part the people living in Vegas appear to be fairly free, secure, and prosperous. Given a fourth option of joining the Courier (well, a good Courier), I'd join them instead, though. I'm fairly certain that, after all they've survived, a little case of possible backstabbing from Yes Man wouldn't be a problem.