The Mentzer sets, also known as BECMI, for Basic Expert Companion Master Immortal. Of course, this is the edition I started with, but actually I never played it much beyond the basic set, changing to AD&D. I didn’t see how great this version of the game is until I started playing it again about 5 or 6 years ago.
It’s the most complete version of D&D, with rules for dungeon and wilderness adventures, Domains and Strongholds, weapon mastery, and adventuring on alternate planes of existence. Characters can advance to level 36 and even beyond, petitioning to become an Immortal (god). Maybe it’s a bit much, and, since few campaigns will run that long, unnecessary, but it’s nice to have these options.
BECMI also has the best adventures published for it. Each set has a series of adventure modules associated with it: the Basic Set has the B-series of modules, the Expert Set the X-series, etc. This are not edited tournament modules that give a wrong picture of how an adventure fits together. Each adventure teaches DMs the rules from the corresponding boxed set. In Search of the Unknown and The Keep on the Borderlands are great dungeon adventures, and The Isle of Dread is a fantastic wilderness adventure.