Metal Slug and Bust-a-Move (Arcade)
The ultimate arcade games. Almost always found paired together, I've spent many a quarter climbing to higher and higher levels in these two games.
Roller Coaster Tycoon 1 and 2 (PC)
I've spent an unbelievable amount of time on this series. It's so important to me that I've transferred all my saved games and custom tracks from laptop to laptop as time has elapsed.