But Bronn probably isn't good enough. Bronn's smart, and it's possible, sure, but just because he says it's possible doesn't mean it's likely to actually happen. Long odds on Daario as well.
I wouldn't say it's terribly long odds for Bronn. It'd be one thing if he'd just suggested that he could potentially beat the Mountain while in a fit of drunken boasting, but that wasn't the case. He gave a thorough analysis of the Mountain's strengths and (albeit much more briefly, by necessity,) weaknesses, described his potential approach, and frankly explained what kind of risk he'd be taking. Since he only did this on the off-chance that Tyrion would offer him something more valuable than what Cersei offered him in counter, it's if anything more likely that he
understated his odds than overstated them, as part of the sales pitch. Tyrion's own observation had earlier judged his swordsmanship to be nearly on-par with Jaime's, and his affinity for ruthlessness and practicality is obviously well beyond. Even Cersei, arrogant as she is, recognized that it was worth trading away a lordship and castle, just to avoid the risk of him going up against Ser Gregor.
Getting rekt when you were a scrublord is no way to judge whether you can beat a badass as a badass of your own. Measure is unceasing.
As nobles and veterans go, and despite wielding a near-priceless and superior sword, Jon's never been a particularly exceptional fighter.
Come, now, you're comparing rando gladiators and the fuckboy of Meereen to a dude who's almost assuredly in the top three for swoleness and general violence on his planet (discounting giants and shit, that is).
The fuckboy of Meereen
was the champion of a thriving metropolis built around constant gladiatorial death-matches. It's not nothing.
So for me I think Daario and Drogo would definitely lose, besides their weapons of choice being incredibly bad for a one on one fight with the Mountain their pride most certainly would get in the way. They lack the mental fortitude since Daario is a sellsword and short of fighting him on the bequest of Daeny I can't see him taking any fight seriously enough until it was too late, believing his skills alone would be enough to down Gregor. Drogo although seeming like a competent fighter is still a Dothraki who are far more skilled while mounted than in any one on one conflict.
If you're just basing that on the show, then that's a fair impression to take away. Book lore gives both of them much more the "kill first, show off later" sort of vibe. (In combat, anyway.)
Everyone else I think could easily handle him, I'd even throw Ned in his prime as a Maybe since we see him hold his own against Jaime before being stabbed in the leg, plus he looked ok against Arthur Dayne for a bit which I think is more than most people could do.
Ned was never really noted for his swordsmanship, either, apart from some lines that get thrown into the show. His fights against Jaime and Dayne in the show
could be written down as just drama, but the better explanation is that in both cases, his opponents had good reason to be trying to avoid killing him.
Young Robert Baratheon could probably do it; he took down Rhaegar, who was allegedly pretty badass.
Hard to say how much of Rhaegar's prowess was really just due to posthumous inflation. When asked, Barristan Selmy is uncomfortable with his description as "a warrior without peer" - if only because, as he explains, luck and circumstance weigh into the outcomes of duels as well as the respective strength and skill of the fighters.
Dude had won a bunch of fights and tournaments, though.