The single ad the Official Campaign has run the most (according to Douge Wead) is Three of a Kind which goes after Romney (and the other two) on all being the same when it comes to policy (which is to say crony capitalist chickenhawks who are in it for themselves not for America).
The other awesome thing about this is the more Three of a kind runs the more every other add against every other contender will also hurt Romney because in reality they really do share policies (remember when Newt and Willard were trying to figure out how they were different on the debate stage? Priceless)
If you want to look at numbers just type Ron Paul plus the other 3 in the google keyword tool. Click on exact on the left side and if you look at the locale column he gets over 1 million searches a month which is 1.5 to 3 times more than the others.