I feel obliged to say power if it meant helping massive amounts of people. As tempting as it would be to say sex (assuming I get to keep my career path since I'm in college and, theoretically, on the path to a decent living), I just feel morally obligated to do the 'right thing' and help people via crazy power. Just imagine all of the people you can help worldwide who are oppressed, poor, sick, generally screwed because they were born into shitty situations, etc. You'd be saving lives and improving lives for millions of people. You could achieve all kinds of feats and become remembered forever in history. If you're religious (I'm not) you could even argue that after you died you could have all the sex you want once you die anyway.
With the power route, is this assuming I keep my g/f (i.e. steady sex from one girl) and my career path (decent living and therefore decent money?). I think I'd choose power even if it meant me being a sexless, poor bastard but being able to keep the steady sex and decent money would be fantastic.
The sex one is flawed too if you think it over. I think some of the reason behind the greatness of sex is that its not easy to get. Your dick can only handle so much sex in a given period of time. Plus you're infinitely more likely to pick up a nasty disease and even die from HIV or something. And what about the people who already get regular sex? If you're already getting it on the regular and are fine with that then power and money look a lot better.
Money is by far the shittiest of the three IMO. I've never been much into acquiring a lot of money or power, I just want enough money to live comfortably and after that its just a pain in my fucking ass paying bills, paying taxes, paying for EVERYTHING. Drives me nuts. I wish I was a kid again and I never had to worry about fucking money. Like Big said, mo money mo problems.
In short, the only reason I'd choose power is to help people. Not that I'm even really into helping people but I'd feel morally obligated after the way you described it... Sex is really tempting though lol.