Did RS-72 ever finish development? If so, that seems like it should be a strong contender. Same propellants as AJ10-190, but twice the thrust and a 24 second ISP increase, could substantially improve performance. Its a gas generator engine though, which could affect safety.
Uprated OMS completed development at Aerojet, flight hardware was constructed and qualified and accepted, but never integrated into STS. So far as I know, there are several shipsets of these engines sitting on shelves. I believe five pairs were completed.