The pre-windows games (MM1-MM5 and Swords of Xeen) are not all that easy to play these days, although one of them (MM3) was IMO one of the best-written RPGs ever from the point of view of plot, puzzles, and game design.

MM6-8 were excellent games in their own right, and there are a number of good mods of these. Probably the best known of the mods are a couple written by a small team headed by Big Daddy Jim (BDJ). The mod of MM7 cures what I thought was a disadvantage of the original game, in which no party could learn both Light and Dark magics. The mod of MM6 is more far-reaching, and amounts to the creation of a new game in the same geographical layout. It's called The Chaos Conspiracy.

As Bones says, MM9 was badly rushed. 3DO was rapidly going down the tubes and could not afford the resources to finish the game before the axe finally fell. TELP (notably Bones) did its best, but some of the flaws are built into the game engine, which is much more difficult to modify than scripting errors. It's a great shame - IMO it could have been one of the great MM games.