Discovery - in the steps of columbus (Impressions, 1992)
For me, the first "discover new land and settle on it" game in real-time! I loved this game so incredibly much, i think i spent months of my lifetime on it back then.
Also, Mega Lo Mania - replaying level after level until i found out how many people *exactly* i needed to just beat the CPU. Also, months of my young life... gone

- Gil