Jaime, Gregor, Sandor, Selmy, Dayne > Robert
Robert in his prime was really a beast. He could swing a huge warhammer with one hand and his prowess with it was legendary, as well as being called peerless and fiercest warrior in the realm by Ned.
Now these would be all very hard matches but I'd bet on him against the Mountain and Sandor as well.
Against the Mountain, Robert would be more mobile as well as more skilled. And those blows from his warhammer would be heavy lol, I think he'd up shattering the armor of Gregor. He could lose, nobody is really safe from Gregor, but like I said definitely my bets are on him.
Against the Hound, he faces an opponent extremely similar to him aside from the weapon. Both freakishly strong, skilled. It could go either way but I'd still bet on Robert more often than not with his portrayal.
Dayne, Selmy and Jaime are at the pinnacle/near pinnacle. Dayne is crazy wanked in the story and is the first character Martin picks in a trial by combat, Barristan is equal to him without Dawn, Jaime is uber wanked as well both in story and by Martin. Their prodigious ability would prevail against Robert. Tough matches nonetheless.