The answer is simple, that is if you want Mitt Romney or Rick Santorum (or even Newt Gingrich or Ron Paul) to be the President. Vote you choice. But, if you're hoping for a brokered convention and if you want to deny Mitt Romney the nomination, the answer is "It all depends on where you live."
If you live in Michigan, Arizona or Ohio--vote Santorum, he has the best chance of defeating Romney in those states and amassing a greater number of delegates. Gingrich and Paul appear to be falling short of the 15% threshhold for delegates in Michigan, so you will waste your vote if you support either of them. Arizona is winner take all, so supporting Santorum instead of Gingrich or Paul will yield the best chance of defeating Romney. And without a doubt, Pennsylvania voters need to support Santorum. He must have a home state win.
If you live in Georgia, Tennessee or Oklahoma, vote for Newt on Super Tuesday. Especially those of you who live in Georgia, since winning his home state is critical to keeping his candidacy alive. Keeping Gingrich in the race is important as a strategy that leads to a brokered convention. Santorum votes in these states will possible lead to Romney victories because of a split among conservatives and Gingrich is best positioned to win in these three. And vote for Gingrich in Indiana also--Santorum isn't on the ballot.
Here's the big challenge: Vote for Ron Paul in Virginia. Let's give him his first win on Super Tuesday. Gingrich and Santorum aren't on the ballot. So it's either Ron Paul, Mitt Romney, or stay home. Staying home is ultimate a vote for Romney. Virginia is proportional, so even if Paul doesn't win, he can hold down Romney's delegate count.
In the caucus states on Super Tuesday, it's important to assess how your fellow caucus goers are voting. Be sure to vote strategically to ensure the largest pool of non-Romney delegates--even if that means voting for Ron Paul. It's not like he's going to win the nomination.
With strategic voting in each state, we can all get a do-over at the Republican Convention in Tampa.