MM6 remains the gold standard in every respect: replayability, scale (I still have trouble finding certain people in Free Haven, despite having replayed the game at least half a dozen times), skill and magic systems, plot. But I truly feel that had MMX not had the client issues it had, it would be more widely considered a worthy #2. Firstly, it's beautiful - really, really beautiful, a big jump up from the MM6-8 engine (let's ignore MM9). It has scale, albeit somewhat different because it's tile-based rather than free world. It has a satisfying, long-term plot, and a sensible skill and magic system. And you can take both Light & Dark within the same party, eliminating the major gripe with MM7 (again, albeit that only one class can take Dark, which dramatically cuts down party choice & replayability). It's at least the equivalent of MM7, although for different reasons.
I've purchased the full back catalogues of MM, Wizardry, Eye Of The Beholder and Ultima through Good Old Games over the years, and was most eagerly looking forward to MM3, largely because of your strong recommendation. But I found that modern systems have spoiled me for going back to the old turn-and-tile-based systems, where you have to take each turn and step separately, randomly encounter 99 goblins, but have a spell that can wipe them all out in one casting, but it still takes five minutes to cycle through the deaths, etc etc etc. So I never lasted longer than about half an hour each time I've tried to start. I played MM2 back in the day and enjoyed it, but this was before things evolved. If I had played them in their era, I'm sure I'd have the same fond memories as you, but I just can't sit through it now.
But MM6 was a ground breaker in so many ways. The only thing close is Wizardry 8, which in some ways is even better, but the sheer scale of annoying encounters and the lack of a real-time option to simply run past them lessens the enjoyment significantly IMO.