For me, MM7 stands head-and-shoulders above the other games in the series.

Anything older than MM6 had severe balance issues. Mages were punished with ever-increasing SP costs as they leveled up (and abilities cost gems). Meanwhile melee fighters received free extra attacks per turn as they leveled (in MM2, a level 255 barbarian could attack 64 times a turn. Insanity.)

MM6 turned the combat balancing tables the other way. A lot of people mocked it as "Magic and Magic". Gone were the multi-attacks as a leveling perk, instead, melee combat could only increase damage a small amount with skill level (and on certain weapons only). Gems as a resource cost were gone, and mages now received some good damage increases for leveling their skills.
MM6 had a myriad of other balancing issues, mostly left over from the earlier games in the series - it's obvious that the players complained, and they were mostly fixed in MM7.

MM7 - the crowning achievement of the entire series. While MM6 had more detail in buildings/architecture and a larger game world, MM7 bested MM6 in every other way. 9 character classes, with elemental/self magic hybrids. Armsmaster skill and grandmaster skill tiers means that knights are no longer irrelevant. Mages also got improved abilities, with the costs of light/dark magic going down.
MM7 also had the most original and unique storyline in the series, with 2 paths - in one path, you finish building a portal to other planets, on the other path, you actually conquer the world. It is also the only Might and Magic game where the main quest isn't about saving the world.
MM7 also had the best character and item sprites. Items and characters in MM6 and also MM8 (3DO was having budget issues when MM8 was being built) looked like they were drawn in Microsoft Paint. In MM7, all of the sprites look smooth and professional. I have yet to see any game that makes 640x480 resolution look better.

MM8 could have been a good game, but 3DO (the publisher) had budget issues and the developers had just thrown out the planned storyline for MM8 due to negative reviews from fans. The option to change your party mid-game was a welcome improvement, and MM8 also further improved combat balance, but otherwise it was rushed, had relatively little content, had a smaller game world than MM7, and also had the most unoriginal bland storyline of any Might and Magic game. It's obvious that MM8 was the "beginning of the end" for the series.
I would say that MM8 is worth playing just because it combines the best of MM7 combat with a flexible 5-character party, but it's nowhere near the legend that MM7 was.

MM9: mostly not worth it. MM8 had mediocre sales, and as a result, 3DO took Might and Magic development away from Jon van Canegham (the original writer of the series) and set up an entirely new team (which had never built a game before) to build MM9 using their new Lithtech full-3D engine. MM8 was heavily criticized because it did almost nothing to improve on MM7's graphics and was considered outdated. In the age of Unreal, Quake, and other 3D engines, Lithtech was 3DO's answer to demands for ever-increasing graphics and 3D quality.
Turns out it was too little, too late. 3DO actually went bankrupt while MM9 was being developed, and the developers were forced to rush it out the door while it was in alpha testing. MM9 was glitchy at best and unplayable at worst, and it was obvious that it was rushed from the lack of endgame content. Supposedly MM9 is playable with community-made patches.