My first M&M was (not counting heroes 2) 7, since then I did all of 'em but MM1&2.
Voted 7 but in fact I can't really decide between 6 and 7.
In 7 I think I like the storyline better, I like the Harmondale feeling, I like the new class & skill system clearly better, like some spell tweaks and dislike others. MM7 is a game I can replay several times in a row every 2/3 years, hail to its replayability.
As you guessed, my feeling is there's less replayability in 6, however I like the music much more, like the environments more, the bigger towns, the bigger everything, the far more complex dungeons, the far more crowded mob packs.
So yeah, mixed. If the question was "You're allowed to play a single MM once. Which one ?", then MM6. But after the answer I would punch you in the nose to remind you you shouldnt restrict my M&M gaming
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― Richard P. Feynman