I really can't narrow it down to just one but for sheer hours sunk into it in my youth, I guess I'd have to go for Pirates! (not the more recent "Sid Meier's Prates!", although that was good too). Spend so long and it never felt old somehow, with the different time frames, diverse challenges depending on your nationality, reenacting famous voyages, all within a dynamic game world. So yeah, there are about 20-30 games that could be my "favourite ever" depending on my mood at the time, but I'm going to go with the original Pirates! on the Atari ST.