I'm of course attuned to Gliscor, it's probably right up there as one of the Pokemon Ash caught that progressed the most (I'd say the best at that was definitely Infernape, however). He caught it as a really weak Gligar that couldn't even fight, and it ended up being one of his strongest team members right when he needed it to be, and accomplished something half his team couldn't. A powerful accomplishment, I would think.
If not Gliscor, I had a soft spot for Torkoal, just because of the potential it had. It did a good job fighting Registeel, even though it lost.