A single level 100 Articuno in Pokemon Blue, I believe. Back in the day, when G/S were still the newest games, a lot of time was spent leveling up all of the Pokemon in the games me and my brother had (actually, it was mainly my brother who did this. He even set a goal to catch all the Pokemon in his Gold version, and get them all up to 100. He never actually did get that far, but he did get an impressive amount up to level 100, only to lose them all due to the internal battery running dry D:), and I just wanted to try and beat the E4 with a single Pokemon. Articuno was one of my favourite Pokemon back then, so I just went with that one. It took a few tries, but after some time, I did do it, and I was pretty happy about it. Never actually done anything like it ever since, but then again, I haven't gotten a Pokemon to level 100 since Silver either :p