Some good answers on here. Every blizzard game for me basically up to starcraft 2/diablo iii. Although I've always been fairly good at those games, they weren't very difficult so I'm not sure they meet the criteria.
There are a few memorable ones from childhood for me:
-Beating every mission of perfect dark on perfect agent stands out. God that was so difficult but so much fun.
-Collecting all 150 pokemon in the original version. Definitely inflated my 4th grade ego.
-Perfecting Zelda:Ocarina of time. All heart containers, secrets, etc. **** yeah.
-Nothing specific but Civilization series (and especially civ 2) in general. So many epic moments/games that you manage to win after 1000's of hours and reloads (yes I reload
)
Currently it's gotta be hitting legend in hearthstone. Soon...