I can definitely agree with you one both :) Although I feel like it's become kind of a cliché in the anime, where he always struggles with the rival in the beginning of the region, and then gradually grows to be more capable in the battles against whoever the rival is, as he gets further around in the regions. I don't know if it's just me, but I would actually like to see the roles reversed at one point. I mean, his rivals are usually completely new trainers. He's a veteran who's been travelling around 5 regions now, and made it into at least the top 16 of every league he's participated in, why isn't he the one who completely dominates each battle he has with his rivals :p But yeah, I would just love to see more growth and character development in his rivals at some point, so it feels more... logical I guess xD But with the current flow of the character development, I reckon some of my proudest moments comes from whenever he beats his rival as well.
My two most proud moments would however be when he beat both the Orange Islands league, as well as the Battle Frontier. His battle against Drake was one of the more enjoyable in my opinion, and I definitely loved the final showdown against his Dragonite. And as for the Battle Frontier, it's pretty much self-explanatory. I mean, I never beat it, so seeing Ash beat it was surprisingly satisfying :p
And with my most disappointed moments, it's pretty much the same as yours, as well as with probably the rest of all who watches the anime. I already explained it earlier, but he just shouldn't be the one losing. I mean, even if Pikachu lost its Electric powers in the battle against Trip, it still has Quick Attack, Iron Tail and whatever else it might have, it should be capable of taking on a mere Snivy hnggg. I'm also pretty disappointed each time he asks about something extremely obvious related to moves, or whenever he can't tell the type of an obvious Pokemon, just by looking at it. I don't have any specific examples of the former, but it's happened often, and especially whenever he forgets the type chart, and as for the latter, I'm pretty sure he got all the typings wrong on the Unova starters :p
So yeah, lots to be disappointed and proud of with him, so I reckon he's a pretty balanced character after all ;)