One thing that occurred to me: In Might and Magic 6 and later (in other words, the MM games that have skill systems), is it feasible to raise a skill to the maximum level? In other words, can you raise a skill so high that the game won't let you raise it further, no matter how many skill points you have?

Or is the skill cap so high it isn't practical (or even possible) to reach?

Maximum Skill level is 60. It is achievable with enough time investment. It requires 1829 Skill Points to raise 1 skill from 1 to max. You get ([Lv/10]+5) Skill points per level and each Horseshoe gives 2 skill points.

E.g., from time to time I do Hive runs. 1 Hive run = 1 level (I am in early 100s). After training is over, everything in Hive is respawned.

Edit: MM7 has smaller world and opportunities, but it has well, which gives infinite gold, exp and skill points... It isn't even hidden.

MM8 has level cap of 200, so you won't be able to truly max even 1 skill without plenty of horseshoes.

Which game does this apply to, and how does it compare to the rest of the series (6+) in this respect?

It might be possible within a normal game, if you focus solely on that skill. It should be possible if you dedicate yourself to it, going far beyond what you'd earn within a normal game.

It isn't feasible within a normal game, however. You'd have to sacrifice pretty much everything to raise the one skill, and by the time you hit 25-30 skill points in a single skill, the rewards just drop too far to be worth it if all your other skills are at level 1.

I added in later edit:
MM7 has smaller world and opportunities, but it has well, which gives infinite gold, exp and skill points... It isn't even hidden.

MM8 has level cap of 200, so you won't be able to truly max even 1 skill without plenty of horseshoes.

This is one of the reasons why I like MM6 the most out of MM6-8 trilogy.

The Hive is a dungeon in MM6. He mentioned MM7 and MM8 separately, so it isn't them.

Edit: Ah, I didn't notice an edit added more data.