While the most obvious thing would be to say Claire because of her useful resistances and the Pokemon that she carries around, I think all of them stand a fairly good chance. I don't imagine Bugsy lasting particularly long though, honestly even with his rematch team, I found him a piece of cake to beat. Overall though even though bug-types can do wondrous things they aren't exactly the legendary type in a Pokemon game. Yes they can probably inflict status but other than that their stats aren't particularly wonderful and they don't have much coverage against the other gym leaders. In fact all of them either resist it or only take neutral damage.
Faulkner, maybe. Flying types are top notch with huge speed and admirable attack. However I'd imagine that they aren't exactly too strong defense-wise and steel-types have no problems coping against them. Also his strategy is to mainly lower accuracy and then heal back any damage up, however this doesn't always work. With Aura Sphere and other non-missing moves being carried around by quite a few Pokemon, it wouldn't be too hard to get past him. Moving on, Whitney. Honestly I think I'd forfeit at the sight of her Miltank. But in my honest opinion, the only reason Whitney was hard in HG/SS was because you didn't have many Pokemon to tackle down her Miltank. You couldn't exactly rely on strategy either as all you had at that time was Tackle or Razor Leaf or whatever your Pokemon carried.
When you rematch her, she isn't half bad. You've prepared against countless Pokemon, you probably have great coverage on your team and Pokemon with phenomenal attack stats or at least a not to laugh at defense. That pink cow won't be getting the last laugh anymore. Plus Normal types can barely do anything without moves of another type. Ghost-types particularly enjoy facing them along with fighting types. Whitney? You were good in the beginning but honestly you wouldn't stand a chance. Even though your Stomp + Rollout combo could easily be beaten by a Female Geodude.
Morty. I remember his Gengar giving me particular trouble. But honestly its playing a risky move running Dream Eater and Hypnosis. The Pokemon can wake up at any given time and Dream Eater will go to waste. Add on the fact that Hypnosis has unreliable accuracy and can miss when you need it the most. Overall he was an okay gym leader but his strategy isn't one of the best.
Chuck, Chuck, Chuck. His Focus Punch strategy wasn't one to my liking. I know that you probably wouldn't see Substitute in-game, but they could of just done so much more with his team. I mean they're the kung fu fighting part of the game! One of the most feared types competitively and in-game. I just feel disappointed honestly. He was a nice challenge and has good coverage but overall he'd be easy to get around. Jasmine, ehhh. Not hard at all and I don't remember any particular strategies.
Finally Pryce. I think he's the winner here. He has great coverage, can hit the feared dragon-types and for anything that resists, just fire off powerful Earthquakes or Psychics from Jynx. Plus throw in the fact that Hail could allow him to abuse Blizzard to its fullest extent is quite alarming. I'd say he has a good chance to be the last man standing with his coverage.
If you couldn't be bothered to read every single word, then my vote goes to Pryce. :p
