The truth is that the presidency is an executive function, and neither senator nor VP is an executive position. Of all presidents who have seen their sitting VPs run for election to the presidency, only Reagan's, Andrew Jackson's, and George Washington's sitting VP were elected (granted that beating Clinton's VP was--ugh--a close-run thing).
Look back over the past generation or two--GWB, governor. WJC, governor. GHWB, coattails of Reagan. Reagan, governor. Carter, governor. Nixon, ex-VP (but his competition was a sitting VP; one of them had to win). Johnson, sitting president. Kennedy, Senator (but his competition was a sitting VP; again, one of them had to win). Ike, renowned WWII general. Truman, sitting president. FDR, governor.
Basically it's a fluke when a non-governor gets elected to the presidency. There is the occasional renowned general (tho Wesley who? is hardly that). So, especially with Cheyney's questionable health, you have to look to the Republican Governors' Association.
Who is the sitting Republican governor who has garnered the most votes? Jeb Bush. I think that makes him the early favorite. There is one woman Republican governor, I believe--but not so's you hear her name every day . . .
Now Senator Guliani still sounds good to me.