In my mind I usually split the Might and Magic games into 3 separate 'eras'. This would be
Era 1:Might and Magic 1-2 Era 2: Might and Magic 3-5 Era 3: MM6-8
It's pretty easy to do this, since all three eras have a specific engine for those games (with minor tweaks between games). Of course this is my opinion. For me, this is a pretty hard question. I was introduced to Might & Magic through my older brother playing the Heroes games. He is a huge fan of their games, but doesn't know much about the core series. I was curious, so I did some research on the games that birthed the HoMM games. The first real MM game I saw was Isles of Terra, and I thought it was amazing! I didn't know much about classic crpg rules at the time(what the heck is AC? XD) but I still liked the idea of a first person world with multiple party members. I got the MM 6 pack as my first game on GOG a while after my first exposure to Terra and played most of the other games. I am a bit ashamed to say that I never has fully beaten a MM game, but I'll get there someday. I still have a soft spot for the cartoony graphics of MM 3-5, but MM 7 is also one of my favorites, because it connects to HoMM 3 so well.
I'd like to hear some other opinions on this. :)

1-2 and 6-7 are my favourites, but it's hard to judge the first and third era against each other.
Not a big fan of the Xeen era from 3-5, with combat so simplified compared to 1-2 that it could just as well have been real time.

MM4-5 are my favourites, but followed closely by 2, 6 and 7.

MM2 is actually the first one that I played, back in the 80s. I remember it being brilliant for its time, and it's probably because I grew up with it that it's one of my favourites. MM4+5 definitely stand out for me though, I must have played through those games about fifty times.

For some reason I didn't get into MM3 as much, but I did finish it once.

I haven't played MM3 yet, or MM7 and onwards. From the ones I did play, I have a soft spot for Xeen, but going back to play MM1 and MM2 I was very impressed with them. I'll be able to do a better comparison when I get around to playing MM3 and Xeen again. Xeen was the first one I ever played so it may be partially nostalgic.

Of the modern era games, I have only played MM6, but didn't like it as much as the earlier games. I still had fun with it though.

I also thought the time travel could have been more interesting. It seemed like I only did it just to visit a specific location, and I would have liked more reasons to explore the world in different eras.

By the way, for the fans of MM1 and MM2, there's an indie developer making some VERY similar RPGs. The first one is done, and it's decent, but it's just towns and a subterranean world. But the upcoming sequel will have a full open world map to explore. I don't think it has time travel, though. Interested parties can read about it here:

http://www.olderbytes.com/genre/swords-and-sorcery/sovereign

I'm not actually involved with these games in any way, I was just interested in them since I enjoyed MM1 and MM2 so much, and figured others here might be interested also.

Okay, a question for you Might and Magic fans. Does gold earn interest in the bank in Mandate of Heaven? I know it did in 3-5.
Yeah, I've been playing 6. :D I loove it!

No, unfortunately gold doesn't earn interest in 6-9. You're right that it does in 3-5 though.

I prefer 3-5, the games I played first and most.

I prefer MM2; a lot of that is nostalgia, being my first MM game (and first CRPG), but it also was one of the best games for depth and graphics during its time, when memory still mattered.

It took me awhile to get into 3-5 and 6+ since they felt so different...but now I am really enjoying them as well! I would say 6+ is my second favorite era and 3-5 is my 3rd favorite, but deciding which one is my favorite era is like arguing whether BG1 is better than BG2...all of them are exceptional!