Hardest gym battle was Black and White against the champion. That was insane. Not only did I chose my roster of 6 based on aesthetics (aka terrible stats) since I was not up to date with the current generation of Pokemon, but in order to “win”, you must catch Zekrom/Reshiram (which I stubbornly did not keep in my roster and I chose to try catching it with a Pokeball), beat N, who has the legendary dragon opposite to the one that you did not catch (Zekrom/Reshiram), and after beating him, you must also fight Ghetsis. Beating all three in a row with sub-par pokemon is really difficult, and while I don’t think you need to battle N/ catch Zekrom/Reshiram again if you lost at Ghetsis, I do know that you must catch the legendary dragon again if you lost at N.
I was just gonna post a similar answer about charmander haha!