MM6 is one of my favorites, too. I always loved it in those games when you finally get flying and can soar over the world--plus, they always hid lots of nice goodies in areas that couldn't be reached any other way.
I agree, though, it was all downhill after 6. I haven't played the earlier games much, though I was impressed with what I saw of Xeen. Massive game there if you combine them all!
I did play Temple of Elemental Evil and didn't like it all that much. It did have a great engine, but the story, setting, or gameplay just didn't really grab me like Baldur's Gate or Dungeon Master did. As I recall, at some point you get a golden skull that pretty much lets you obliterate anything with impunity.