The second game I ever 100%ed was Arceus and I still can’t stand the thought of picking it up again, years later. The first game was Horizon Zero Dawn, which is still fun.
Baldur's Gate 1 & 2...the exploration of the wilds while so vulnerable in the first game is something I'll never get bored of while the second game just takes it to a whole other level with your power growing so immense. They still totally hold up with the extended editions!