Probably Wade or Colfax. The other high-profile Republicans at the time were all unlikely choices: William Seward had sided too much with Johnson during the latter's Presidency, Edwin Stanton and Thaddeus Stevens were dying, and Chase was functionally a Democrat on most major issues except slavery and the war (to the extent that he was seriously proposed as a compromise candidate for the Democratic nomination in 1868 when that convention was deadlocked between Hendricks and Hancock).
I'd bet on Colfax over Wade, since historically he won out for the Vice Presidential nomination: Wade had more support in the early ballots, but less than a majority, and Colfax seems to have been the second choice of most of Fenton and Wilson's supporters.