I think I enjoyed MM6 more than any of the other Might and Magic games except MM3. For me, that was the pick of the lot. OK, the game engine is now positively antique, but I still think that it's unbeatable in terms of the layout and puzzles. There really is never a dull moment in it.
MM4/MM5/WoX were pretty good, too, but some parts are seriously boring - the Red Dwarf Mines to mention but a few! Too long, too much of the same thing, not enough combat. It really comes into its own, though, when you start tackling the WoX dungeons - the ones for which you need access to both games.