Hard to say... The problem in gauging Robert's ability is his unusual choice of weapon. The warhammer is slow, clumsy and hard to defend with. It's also incredibly effective against plate armor, even more so because of Robert's awesome physical strength. That being said Robert had the advantage at the Trident against Rhaegar because they were fighting on horse, and in a river. Mounted combat is fought in passes. They don't just park their horse next to each other and hack away. The run torwards each other, exchange a blow or two, spin around and do it again. The warhammer is perfect for that because it doesn't need precision, and even glancing blows can do damage. On the other hand the sword is robbed of its advantages. It's speed and precision don't mean much because you're not long enough in contact with the enemy to work the vulnerable parts of plate armor, and glancing blows are useless. That's why knights use lances instead of swords when mounted. I believe that fight would have gone differently on foot...
I'd say Robert was a very competent warrior, way above average, but not quite top tier. Rhaegar was also a very acomplished tourney knight(jouster), and while I'm sure he was a very capable swordsman, he wasn't top tier either. We don't really hear of any of his exploits or prowess with a sword, just his jousting ability. Also the Trident was his first taste of battle, unlike Robert who had been fighting the whole war. All of these factors created a perfect storm scenario for Robert that I don't think can be replicated against top tier warriors.
So I reiterate, Robert was very good, but not quite among the best. Give everyone their choice weapons and he can be defeated both on foot(by Jaime, Barristan, Sandor, Bronn, Oberyn etc.) and on horse(by people who are good with a lance like Loras Tyrell, the Clegane brothers, Jaime, Barristan etc.).
@ Robert running over his dead body to get to his objective though.
