"As far as I can tell, their records are mixed on that issue. But I haven't researched it in depth."
Nor have I. All three are grabbers to some extent. John McCain was a supporter of the awb. Rudy has ben a fan of "gun control" for some time. And Mitt Romney hasn't been much better, although he has been trying to recharacterize himself of late.
About the "best" of the top three, and I use that term loosely, would be Mitt Romney. Frankly, the top three choices in my view are nonchoices, and are only distinguished by the fact that they're better than what my party is touting.
We're in for a long four or eight years on gun rights unless the GOP or Democratic party is willing to run someone approaching a conservative for the Presidency. And that looks less likely over time. Our best bet of late seems to be working in the state legislatures as well as (ironically) in the courts.
There is also the fact that neither party is particularly eager to take on the gunnies politically, so I wouldn't expect dramatic changes absent a complete take over by one party or the other. Even then, I wouldn't be too pessimistic as gunnies are about the only conservatives who have figured out some winning formulas for practical conservative politics. From a cost/benefit point of view, it doesn't make much sense.
There is the possibility that former U.S. Senator Fred Thompson (currently starring on "Law & Order" as a D.A.) will run, and his record on guns is good.