I would be honored if the following mods were featured in this list:
Isles of Terra Monster Spawn mod
World of Xeen Monster Spawn Mod
As a cursory glance at the changelogs available via the two links might indicate, they are "Monster Spawn mods" but are so much more than that. In short, they attempt to add a "Might and Magic 2" dimension to these games, so that you never run out of combat opportunities. An attempt is made to balance the Ranger/Druid spell system with the Cleric and Sorcerer spellbooks, and the Isles of Terra mod includes a couple of bugfixes as well.
Obviously, I tailored these mods to cater to Might and Magic combat aficionados, and they may be tedious to those who'd rather play the games for the story. However, there are numerous edits that may interest casual gamers as well (such as the opened Easter egg in MM3).
Of course, I think it's great fun to go back a couple of areas to kick the crap out of monsters that used to terrorize my party when they were a few levels inferior. In my opinion that's helps drive home the feeling of accomplishment as your party gains in power.