And you guys haven't mentioned the wells that--- with Beacon--- for me come close to breaking the game about mid-through. Obviously, by game end, they are proportionately less so. But Lloyd's Beacon was too useful in MM6 for several of the middle quests.

I didn't bother with wells in my 1st playthrough, and even now I only use them situationally, when I am in their respective zone. I like that MM6 gives various ways to "break game", but I think clear winner is MM7 with its Well in Eofol: you basically hold down space button while recasting "Protection from Magic" from time to time, while enjoying flood of Exp, Gold and Skill Points.

Those don't sound as good as the wells in MM2 and MM3. In MM3, a combination of the wells and the "DOE MEISTER" cheat can get beginning parties (temporarily) up to level 251, allowing you to take on the dragon dungeon early in the game. (Or, if you can get Implosion, even get the item that lets you warp to any map.) In MM2, a combination of wells and a few other mechanic (like the fact that recruitable characters already know all their spells) allows you to kill the Cuisinarts with a beginning party (I actually killed them as my first victory once and took a screenshot that I posted in an older thread here).

The Xeen games aren't as breakable, but with the help of wells and powerful magic, it is possible to reach and clear the Dragon Tower early. (The 20 million experience you get on the top floor is significant when your level is still low.)

Are any of the "modern" (6 and later) games breakable to this extent?

8 was the only MM that I ever felt truly was out of wack when certain choices were made (Dragon, Five member parties, Cauri)

Doe Meister is dirty cheat as I don't even remember it being referenced in game. It is like hex-editing file to add few millions gold. Same with making slave party and raising bank funds to 2 billions. Unlike some other games, MM games allow you to reach godlike power and wealth via normal gameplay.

All MM6+ have no stat caps, that is enough to eventually "break" the game (I hate word "break", it is just normal results of power growth). There is secret dungeon in MM6 (NWC dungeon), which is basically low level cheat and is better saved for 2nd+ playthrough.

MM7 has well in zone with titans, on which you can keep clicking to gain exp, gold and skill points, it has infinite charges... However, you need to do some preparations to reach that well, as you will have to go through caves with very nasty creatures and you need Protection of Magic GM (iirc Master wasn't enough to prevent sudden deaths).

MM8 has dragons as party members, but it felt very unfinished. E.g., there should be training center in Regna, which would allow to train without limits, there is only 1 promotion, etc. All in all, it is better to save MM8 when you want more of MM but already played previous ones.

In general, MM6+ are as "breakable" as previous games in sense that they don't limit you by super low level caps, static stats, same gear for whole game, etc. and you will end up raining death on all monsters. MM6+, however, plays quite differently, I'd advice to play them first time without any intent to "break" them asap and without using any guides (especially which teach exploits). Even if you will "break" them by some guide, what good will it do? You won't know where to seek teachers, their requirements and such, which means using more guides, and in the end playing the game through guides, instead of exploring world on your own.

Out of interest, who even noticed that Pathfinding Teacher in Vertigo (World of Xeen) says that he will teach you Pathfinding for 2500, but in fact he charges only 2000? :)