Seweryn: So what M&M would you suggest as the first game of the series ?
The recommendation of MM6 or MM7 as a first game isn't a bad one. But in case you are curious about the other games:
MM1 and 2: These are VERY old, but still quite fun if you can get past the old interface. First-person party-based dungeon crawlers, they are entirely turn-based and party movement is done "step by step" on a grid map, moving one square at a time. A modern example that works similarly is Legend of Grimrock, but MM1 and 2 are turn-based instead of real-time and have battles on a separate screen. In MM1 especially, part of the challenge is making your own maps and navigating traps designed to disorient you.
MM3-5: Still turn-based and grid movement, but greatly improved graphics (although still 2D) and now combat happens during exploration, with enemies moving around on the same maps you do. This makes the games much closer to Legend of Grimrock in how they play, except they're still turn-based. I personally like MM4-5 the best (they are combined into one huge game), but opinions vary on this. These games tend to be a bit easier than the others.
MM6-8: Now real-time with a full 3D world, but combat can be switched to turn-based at the touch of a button. These are the games that gave the series a reputation of huge armies of enemies and some grinding. I've actually only played MM6, not any farther, but I did find that game could be a slog at times. Still, exploration is fun and there's lot of potential for character customization and building a powerful party.
In general, the entire series maintains the "dungeon crawl" style, meaning it's all about exploring, fighting, and solving puzzles. There isn't much conversation, quests are simple, and there won't be any big moral choices like you might find in the other RPGs you listed. Even so, I found the premise / stories of the games to be surprisingly interesting, and in MM6 and onward the stories are intertwined with those of the Heroes of Might and Magic games also.
EDIT: All the games do a really good job of providing diverse locations to explore. Even the oldest games made the world feel like a varied and interesting place, with a distinct personality to each dungeon or other location that made them all memorable. That's one of my favorite things about the series.