Leinart and Peterson have the best shots, and I think Marshawn Lynch has the best shot as an underdog.
Don't be so quick to think Oklahoma will be any less strong. Bob Stoops has been arguably the best recruiter in the country over the last half decade, and they will be restocked nearly as strong as they were last season and ready to go. Peterson will be in the running for the Heisman and Oklahoma has a good shot at smelling roses in January.
I think Reggie Bush should be the top contender in 2006. He will be a senior, the whole offensive line should have two years as starters under their belt, making the running game their strongest option. So no Reggie, at least this year.