Last week's episode took the cake for me. Tho Ash vs Cameron is a close second and previous champion, since it was a mess in every sense of the word, i don't even know where to start honestly. Ash vs. Faba and Ash vs Hau were pretty bad aswell, with Ash vs Faba being treated as a literal joke and Ash vs Hau being extremely unfair to Hau in my opinion (he genuinely felt like the strongest trainer for the vast majority of the fight).
I don't think Ash vs Kukui is the worst, but i do get where jane is coming from, i don't think she is obligated to like that battle just because others praise it, as it had no stakes, felt dragged out not really in terms of how many episodes it took but how slow some of that scenes felt, didn't exactly inspire any emotion, the Incineroar battle was really underwhelming and the strongest threat and most interesting opponent in that fight wasn't even a pokémon that he trained, acting more as a battle of it's own than part of Ash vs Kukui (hence why i would much rather praise Ash vs Tapu Koko, than Ash vs Kukui). That said, it did have nice animation and it's obvious they put the most effort on it, tho i still think Ash vs Gladion was probably the best fight in that tournament, as it didn't feel dragged out, there were actually stakes and it felt more emotional as it was the first time Ash won the League, even if they could have made it more by actually including something like a scene where all his companions were watching him.
Overall, Ash vs Alain was probably the most hated when it came out, mostly due to it's outcome. Personally, i thought it was one of the best fights, but i do have my grips with it, like them not presenting at least one more pokémon before, other than Metagross and his ace Charizard. It's nice that they kept some things a surprise, but we didn't need to be that surprised.
There are others that i might've forgotten to be honest, probably due to how boring they were (to the point of making me fall asleep). BW battles were pretty dull (and Pikachu vs Snivy was just ridiculous), tho i think Iris' battles were pretty nice for the most part, even if her goal progression wasn't exactly the greatest. JN has a serious problem of trainers one-shotting (or nearly ohko-ing) everything, so i can't say i found a lot of those fights interesting either. Other than that, Morrison's battle was extremely annoying, as his personality was unbearable and his refusal to attack Ash made no sense, considering he certainly must have fought other friends by that point and he didn't even know Ash that well to begin with lol