My two cents
Rampage had a longer career and a better resume, but Prime Romero beats prime Rampage
Except that KO loss, we haven't seen Romero badly beaten. He lost 3 close decisions.
Rampage was KOd by Silva and Shogun, submitted twice and lost other fights.
Sure, he had better competition, but I believe Romero in his prime would have been better vs Rampage's competition.