Santorum with an A-, Romney with a B-, Gingrich with a D, and Paul with a D-.
Given that Romney and Santorum are presently in the lead, this is fairly encouraging. I'm also inclined to believe that Paul is likely to be slightly better than a D- in practice (for one thing, he'd be disinclined to meddle with the states deciding to enforce immigration laws on their own). Attrition through enforcement plus EVerify is getting quite a bit of attention in the debates and scores well with the voters. The genius of EVerify is that it lays the smackdown on the businesses that knowingly hire illegals, who are decidedly harder to paint in a ridiculously sympathetic light than the illegals themselves. It is for PR reasons more likely to be effective and sustainable politically. I still look forward to the sequel to Operation Wetback though, but I don't see any of the candidates as likely to do anything quite that sensible.
Santorum has also been talking good sense (read heresy in the MSM) on education lately
Given that his kids are homeschooled, it's not too surprising that he's sensible on the issue (read, recognizes that the educational apparatus is a major node of the Cathedral and primarily an engine of indoctrination).
Ron Paul remains the only candidate likely to even try to get the debt and deficit under control (Romney would likely significantly reduce the deficit but the chances of him getting it to zero given what that would require are basically zero, which means debt/GDP over 100% is here to stay).