Robert is not low imho, even be in that list is a great achievement, considering who are the eight before him
I copy-paste it again to keep perspective
1 Arthur
2 Barristan
3 Jaime
4 Hound
5 Oberyn
6 Mountain
7 Garlan
8 Jon Umber
9 Robert
10 Loras
1-2 are said to be absolute top, 3 is the current best boy at some point of the books, 4-6 are monsters and 5 is the one that could have defeated one of them.
7 Garlan is said to be a very very vey good swordman, better than Loras who was elite too and without is his most evident weakness (being light)
8 GreatJon is the Zangief of that universe, for the characters we seen strenght wise is likely second only to the Mountain. Jaime thinks he's one of the strongest men around/worth to fight
GreatJon was essentially a bigger/heavier/stronger Robert
Then you got Robert, no shame have them before him i think
In his prime he was strong af but his only greatest achievement was defeat a great-but-maybe-not-top-10 knight (Rhaegar) and even there he got wounded... i think GRRM wanted show they were even matched, but Robert was the more motivated and best dog that day
Hound high, because hound is a monster, imho the closest to Jaime's spot... i still gave #3 to Jaime due talent/technique/speed/brain, but still Jaime in his own mind is impressed by his speed and aggressiveness, all while being a VERY big and muscle man
Jaime himself see both the Clegane as part of the very few men that could beat him in a fight
If was'nt for the Oberyn fight i will have done it #3 Jaime #4 Mountain-Hound