depends on the space, with places for the ram to go that the angel can't a 20 gallon might do. but then that is just my exp with my laid back angel.
when the ram was still around he was the most agressive out the the trio of angel, betta and ram. that's be cause of the personality of my particular fish, usually I would put it the other way around with the ram being less agressive than the other two.
if no hiding places I'd go much bigger than a 20 though