Of the 3, my personal favourite is MM6 by the length of a street. MM7 is considered by some to be a better game (and I can see why), but my strong dislike of any one party being unable to access both Light and Dark magic biases me against it.
I did enjoy MM8. It's a much smaller game than either MM6 or MM7, and much more handleable in consequence. You only actually create one character; the other 4 are hirelings that you can change if you wish, although if you do that, you have to remember that as soon as you take a hireling into your party, you start investing experience in them. My own personal preference is to start with low level hirelings and keep them to the end of the game. I did play one game in which I deliberately set out to get the strongest hirelings as quickly as I could, but for me, it suffered two disadvantages. First, the game was so easy it became boring, and second, I missed the pleasure of seeing my characters promoted.
My own feeling is that if you lost interest in MM6, you would also lose interest in MM7, which IMO is a more difficult game. So I would try MM8. It is unquestionably the most "accessible" of the last 4 MM games, and has the advantage that no bad bugs were found and no official patches were ever needed for it. I don't know what ops system you're playing under, so I can't advise you on that.