Legend of Zelda (NES) - To complete the second quest.
Super Mario Bros. (NES) - Also to complete the second quest.
Castlevania (NES) - Actually I'm not sure this one counts. After beating this game, I tracked down the MSX version (Vampire Killer) and I beat that game. I loved Castlevania, but Vampire Killer was even better - more challenging (no continues) and I actually liked the idea with finding the keys and the different paths.
Metal Gear (NES) - Again, I tracked down the MSX version after beating the NES version, and again, I thought the MSX version was superior.
Dragon's Lair (Arcade) - You could get a lot of playtime out of a quarter (actually I think it was 50 cents) once you knew how to win. Plus, you got to impress everyone who was watching.
Adventure (Atari 2600) - Great game - different every time. Still love this game today!
Zork: Grand Inquisitor (PC) - Amazingly faithful return to the world of Zork. I loved the nostalgia trip so much, I had to return. Kudos to Activision for such a great Zork game, but damn them to hell for not following through on the rest of the trilogy. I wish they would just sell the Zork license to someone who might have an interest in it - because Activision sure doesn't seem to ever plan on touching this license again.
Luigi's Mansion (NGC) - The game was so short but enjoyable, that I played through again on the harder level.
Mner 2049'er (ColecoVision) - I had to see if I could actually beat the game again!