My top choice was Herman Cain, but sadly, he is not running any more. Of the remaining candidates, Rick Santorum is at the top of my list. If he weren't running, I'd have to go Ron Paul, even though his foreign policy scares me. If Paul had a different foreign policy, he would be my top choice, or very close to it (although I do disagree with him on some other issues as well).
I don't trust Romney -- he was a liberal for far too many years, and (imo) became "conservative" only when it was politically expedient to do so -- and that about sums up his character, imo -- political expedience, rather than character and backbone. Besides, I think he is least likely to win against Obama, because he'll be like McCain, only worse -- even less likeable, and also bland. He won't get conservatives fired up and lining up to go to the ballot boxes.
Newt... he's an excellent debater, but I think he is too much of a "say what I need to get elected" kinda guy (like Romney, but not quite as bad, because Newt does at least have a conservative background and record). But he seems to be pandering to get elected. I think he would be the most effective President, because he knows how Washington works and can get things done... but I'm not sure if I'd always like what he could get done. Plus, he has character issues, and I don't fully trust him. If he can't be trusted to take his marriage vows seriously, why should I trust him to take his Oath of Office seriously?
Any of them would be better than Obama, but here's how I rank them:
Who I'd vote for -- Santorum, Paul, Gingrich, Romney
Who would be most effective as President -- Gingrich, Romney, Santorum, Paul (although perhaps I should bump Paul up a bit, because he'd veto probably 90% of legislation Congress passed, and that's probably a good thing, and can be a measure of effectiveness)
Most likely to beat Obama in a general election -- Paul, Gingrich, Santorum, Romney
Paul gets much of the youth vote, as well as brings in the most liberals, libertarians, and independents, without losing too many conservatives -- his anti-government, pro-small government message will resonate with many of us, and hopefully his libertarianism on social matters won't turn off too many
Gingrich is a good campaigner and speaker, and he can get the conservative base fired up (even if it is somewhat empty words)
Santorum is rock-solid conservative, so may lose a few independents and centrists, but he's somewhat small-government, so may keep some libertarians; but the biggest thing he has going for him, is his ability to keep and solidify the conservative base, more so than Paul and certainly more so than Romney. His biggest problem is that he doesn't generate much enthusiasm from his manner and mannerisms. If he could change that (add a dose of Herman Cain, for instance), he'd probably be better even than Gingrich in this matter.
Romney is bland -- he can't get people fired up to vote for him (he's even struggling in the primary, when he's outspending his opponents right and left -- how much worse will he do when he's being outspent by the Obama machine?), and that's a huge problem. McCain's problem in 2008 was that he induced a yawn in the conservatives, and they didn't so much vote *for him* as *against Obama*. That doesn't win elections. From my understanding (and mostly memory) of past elections, the winner is the one who generates the most positive -- the most people voting *for* him; and the nominee whose best selling point is, "Vote for me, I'm not the other guy", loses every time. It happened in every election in my memory: Clinton/Bush, Clinton/Dole, Bush/Gore, Bush/Kerry, and Obama/McCain, but particularly Dole, Kerry, and McCain.