The history of MM6 and its main patches is as follows (and I can't remember all of it!):–
There were assorted minor bugs in the original v1.0 game, but the major one was the Gharik's Lab bug. This meant that if you used the key to open Gharik's lab, and then left without completing it, you couldn't get back in. There was a flaw in the saved position, because it saved the position with the main door closed and not open. You either had to complete the lab on one visit, or else the much easier solution of setting a Lloyd's Beacon in the lab.
The first official patch cured that (I think!), but did not cure all of them, and I can't remember what bugs still remained afterwards.
The second official patch cured more bugs, but introduced two of its own, in that it caused two doors unopenable, one in Castle Kriegspire and one in the Shadow Guild (in the Frozen Highlands). This blocked one local quest (in Castle Kriegspire), but only under one fairly rare set of conditions did your party really need to solve the Shadow Guild.
Mok's (unofficial) patch was mainly intended to make the game playable under the WinXP operating system, but it also cured those two bugs. I believe Grayface's (again unofficial) patch also cured both.
I've always played the original unpatched v1.0 game.
There was one major bug in MM7, the Lich Jar bug, which could be cured by the official patch as long as your game had not progressed to the point at which the bug had struck your game. There was never any cure once that had happened, although you could use an editor to bypass the effects of the bug.
TTBOMK, MM8 had no major issues.