If it stays in the first round then Leben by KO. Reason: I haven't seen Riggs beat anyone significant in the MW class.
If it goes 3 rounds, then Riggs by decision. Leben has no stams and gets careless with his hands. Riggs has heavy hands and fairly good technique. Not world champ technique but at the very least better than Leben.
Either way, a very nice stand up war if it happens.
Fedor sleeps with a night light. Not because Fedor is afraid of the dark, but the dark is afraid of Fedor.