Of the main three mentioned -- Bayh, Biden, and Kaine -- I think it'll be Biden.
Of anyone outside the main three, I think it'll be Sam Nunn or Bill Richardson. This is mainly because I think Obama will be looking for someone with strong foreign policy credentials.
Of the three I mentioned, I've been saying Nunn but I'm leaning more toward Richardson with Nunn a close second.
Tim Kaine, unlike Bayh he can help add electoral votes into Obama's column.
I think you're overestimating Kaine's impact. He's not as popular as Mark Warner and the Richmond paper had a recent poll showing neither he nor Eric Cantor would have much of an impact on swaying VA one way or another.
