Tradesports gives us two different odds - the odds of any given candidate winning her primary, and the odds of any given candidate winning the presidency. I believe that (correct me if I'm wrong), we can apply the definition of conditional probability to calculate the probability of any candidate's winning conditional on winning the primary to see who is the most "electable." Here are the results:
for the Suckpublicans:
McCain: 50 (!)
and for the Suckicrats:
Gore: 70.4 (!)
I leave analysis of these numbers to my far-too-long-blog-starved readers.