The three compilation sets that I know are:
MM6 Platinum edition — contains MM6-9
MM6 Millennium edition — contains MM6-8 plus World of Xeen (MM4-5) and Swords of Xeen (the fan-designed (fan-made?) game that New World Computing judged good enough to publish)
MM6 Limited edition — contains MM1-6 plus Swords of Xeen
The MM6 version in the first two is v1.2, which means you'll have to apply either Mok's patch or Greyface's patch for it to play properly.
The MM6 Limited edition (which is the one I have) contains the v1.0 game. This has its flaws, but I find it perfectly playable as long as you are aware of the Gharik's Lab door bug, and how to play the game in a way that avoids it.
The versions of MM1-5 are, so far as I know, all the latest versions of the games, and I've not found any bugs in the ones I've played. However, the Swords of Xeen in MM6LE is still bugged, and I have never been able to complete the last dungeon in it, although I found a site once that showed a speeded-up playthrough of it.