I have a few. FYI, I'm not a very good gamer.

Beating the Elite 4 and Champion Cynthia on Diamond
Okay, this was the first time I've ever beaten the Elite 4. And I owned Fire Red, Ruby, and Silver before this. My attention span is really short. What happened was that I was waiting for someone to pick me up one night and I was getting bored. I've already tried beating them three times, saving before I went in that one-way passage, and only got as far as the third person with the Fissure Wiscash (that hit each time, FTF 30% accuracy). Each time I rearranged my team, got rid of unneeded HMs. So on what was supposed to be another trial run, with very limited supplies, I got through to the Champion. I thought I was going to stop there, with the Garchomp and all. The only defense I had against that was my Empoleon with Ice Beam. But Giga Impact missed, I got a free shot, and I blitzed through it and the others. With a Charizard (traded), Empoleon (my starter), Torterra (traded), Lucario (from egg I got on Iron Island), Spiritomb (traded), and a Luxray (mine). All on L50 when I went in.

Getting a score of over a billion on Pokemon Pinball
Another feat I accomplished because I was bored. It was my brother's game and he was pissed when he saw the score. He must've spent hours trying to beat it. Too bad, because he can't beat me at pinball! Hahahaha! Too bad there was something wrong with the internal battery. It erased all the data.

Beating Super Mario World the long way
This was the first game I beat, at about eight years old, but when I got the GBA port I wanted to get all the goals. No Star Road shortcut. I took the hard way. And I'm so proud.

Beating Donkey Kong Country
The first game I remembered beating the hard way. That mattered, at least. I think I was about ten. King K. Rool was a bitch. Man, that was a great platformer. I still remember thinking, "Those credits are fake! Holy crap, he's back up."

Anytime I beat my brother in Brawl
My brother's good by my standards. We usually do a stock Brawl with two CPU characters set at 5. We usually end up facing each other with equal lives. I have the best chance of beating him when I'm using Yoshi.

Okay, one more for now.

Fighting in Twilight Princess
This is only up here because it's enjoyable. First up is the horseback battle with the boar-riding goblins. Just ride alongside the big guy and whack him with my sword yelling, "This is the Hyrule police! Pull your ass over now!"

The Morpheel battle in Lakebed Temple. Something about stabbing a monster in the eye while hanging onto him...

And anything involving sniping. That's always sweet, especially seeing them wobble and fall. Head shot!