First MI: Secret of Monkey Island at 5 years old, first video game.
Favorite MI: Monkey Island 2: LeChuck's Revenge. It retains much of the first game's innocent charm but adds in a delightful dose of darkness. Scared the hell out of me a couple times as a kid.
(Second favorite - SoMI, Third favorite - CoMI, Fourth favorite: ToMI, Hate - EfMI)