For me, I'd have to say Old School X-COM (UFO Defense, TFTD, X-COM Apocalypse to an extent). I love them in a way that I just do not enjoy NewCOM, but the interface was quite possibly based on the recovered works of a 13th century scribe, and is very difficult to get used to for an unfamiliar player.
Donkey Kong Country is a series I speedrun every few years. Probably the nearest and dearest to me, off the top of my head. Ocarina of Time would be another one. Lost so many hours to that, the swordless link glitch, and a gameshark.
Fun fact, you CAN find the Triforce in OoT. You just can't do anything with it. You gotta float over Zelda's Courtyard, and it's just sitting outside of the clipping zone below the grass.
Definitely agree with OoT, loved it, but it confused the crap out of me as a kid. I had no clue what I was doing after getting the master sword.
I remember absolutely HATING the water temple until I got a little older. After that it was like "oh...yeah, yeah, raising and loweing water levels, I get it"