I'm afraid the ones who get the most press time are going to be Rudy, Pataki, and McCain. Those are all guys that I'd rather not have as president. I'd like to see Jeb Bush make a run at it. He's done some good things down there in Florida. If national security issues continue to heat up (Iran, N. Korea, etc.), Condi would probably be a good option (especially if she was going against Hillary). Chuck Hagel is going to go for it, though he has to turn back towards conservatives if he's going to stand any chance in the primaries. As Kerry is going to find out, it's very difficult for Senators to be elected president because of their long voting history. No matter if it's conservative or liberal, there's plenty to attack.
If I could pick a dream ticket right now, I'd go with Jeb Bush/Condi Rice. Unless something very serious happens in the next four years, election focus will turn back to domestic issues, which is Jeb's specialty. Obviously, there will be a need for the national security front, which will be Condi's purpose.
I guess it all depends on the road the Republicans go down. The RNC last week was evidence that we are willing to pedestal our socially-liberal members. If we begin constructing our platform to keep these people highlighted in our party, then it will be a Rudy/McCain ticket. However, if we maintain Bush Conservatism, then a ticket such as Bush/Rice is more likely.
There's no doubt it's going to be Hillary in '08 for the Democrats. I don't yet have an opinion as to how difficult it will be to beat her.