Never really got into the sequels as much, HOMM3 always felt like a local maximum.
Fun fact: one of the modded scripts for HOMM3 was written by the developer of Dominion: https://opinionatedgamers.com/2012/05/11/the-art-of-design-i... and https://youtu.be/i6Uc1-AR4rM
Unfortunately I'm no longer actively participate, but project is still going strong.
Good sides: it's fully client-server, AI implemented via pluggable libraries while all saving / networking implemented using serialization based off Boost.Serialization.
If you're picking it up again, the HD plus mod lets you play on higher resolution screens and adds keyboard shortcuts for things like splitting a creature stack in half. Highly recommended.