The rest of this is entirely my opinion.
In my case, it goes like this (best to worst)- Mccain, Obama, Frist, Hillary.
I have a huge amount of respect for Mccain. Not only did Bush jr. blindside him in the 2000 primary, but he also stands up for what he beleives in, even if it's bad for his image as a republican (for instance, his denunciation of Jerry Fallwell, his stance against foreign oil, etc). He would definately be the one I would vote for.
Next is Obama. Many have said that he's not a good candidate because he's inexperienced. This, I beleive, is his best attribute. He still plays politics with his soul, not just with his party's beleifs in mind. Plus, he would be an excellant choice for the first black president.
After that is Frist. The only reason Frist is here is because (in my humble opinion) he's not as bad as Hillary. Frist is a typical politician; he's better than Bush, but still definately not the best choice.
Lastly is Hillary. Hillary is easily the worst candidate there. She latches onto whatever issue is popular in order to ride it to office. For example, she totally behind the "Hot Coffee" related video game fuss. She was trying to push the issue to make Grand Theft Auto for 18+ audiences only; what she didn't understand was that all the hubbub she was created would've only led to the game being unavailable to 17 year-olds, as opposed to just 16 year-olds. She didn't even do the research on the issue to discover that the only difference would be a single year of age. She flip-flops on issues like a certain other ex-nominee. She compared Jesus and "the good samiritan" to illegal immigrants in a recent speech in a thinly veiled attempt to win over the "religious voter" bloc. She is the worst possible candidate of the four.