Jon casually mentioned that Robb was better growing up while thinking back fondly on his dead brother. That doesn't mean Jon hasn't surpassed him in the years since they used to spar as kids, which he has.
Loras was an extremely skilled tourney fighter who had little experience in actual battle situations. Plus he is was a Knight of Summer and Snow would destroy him.
Mountain is a brute. Skilled to be sure, but if Oberyn could beat him rather easily, Jon could too. He took down Styr after all.
Brienne - great fighter. She won a few small skirmishes in impressive fashion and beat an infection riddled Hound. Still taking Jon based on resume and the eyeball test.
Oberyn - Close fight but I'm partial to Valyrian Steel over wooden spears. In a 'fair' fight I'm taking Jon. But with poison involved all Oberyn has to do is tag you ONCE. A guy that skilled can get single a hit in against anyone.
The Hound - He's a beast. Size, speed, power, skill... He has it all plus pure seething rage driving it. In his prime pre leg injury, Hound by a hair though it's really too close to call. If they fought today Jon takes it due to having increased mobility
Barristan/Jaime - legends in their prime and consensus top 5 all-time. Jon isn't there, YET. But even according to his nemesis Ramsay he's already starting to build his legend has a sword fighter. Though if Ned could at least hang with Jaime I see no reason Jon couldn't. Not saying he'd win but it would be a fight.
Tl;Dr - only 2 people you listed are clearly better than Jon Snow