Richie vs Ash. Honestly I have to say it is the worse battle I have seen. It was terrible how they handled it. Not to mention they have never had a rematch.
Another one would have to be Viridian City gym. Team Rocket vs Ash. I didn't really like the battle. It was no different then half the battle with the Rockets in all of Kanto. But I guess they did make up for it in Unova.
Now I just want to see a real battle between Ash and Giovanni.
Eh, at least their loss was better than their first battle in Episode 3, where they lost easily to a rookie, near-dead Caterpie despite having the advantage in every way, not to mention Pidgeotto was definitely fresh during that time.
As far as Ritchie, I'd disagree, I'd say Cameron was the worst, largely because Ash lost to such an idiot he made even Unova-era Ash seem smart (and you know how idiotic Unova Ash was), which reflected very badly on Ash (and quite frankly, I think the fact that Ash even decreased in rank at all was just horrible as it was). Could the Indigo League have been handled better? You bet. There definitely needed to be at the very least a postponement of the match to ensure that Ash and Ritchie were both up to task. But at least with that, you could blame Team Rocket for Ash's handicapped status and thus his loss. With Cameron, you just have to blame Ash himself for losing to a guy who clearly should NOT have won at all.
Regarding the topic, I guess I'll say... well, the Hoenn Gym Leaders barring Brawley and possibly Roxanne. I constantly think whenever reading up on their battles "...why are Gym Leaders considered pros again?" At least with the Kanto and even Johto gym leaders, they actually WERE skilled enough to not lose to rookies on a regular basis, whether it's the trainer himself or his Pokémon. I guess the Hoenn League is pretty underwhelming. I don't see how people could even love AG as a whole and especially the Hoenn League, when it literally didn't even have any apparent indication that Ash actually did better (he got the same rank as in Johto. That's not exactly a sign that he improved).