In my opinion, the biggest revolution in the series was from Heroes 1 to Heroes 2. Judge for yourself: the spellcasting system was radically transformed, secondary skills were added for the first time, and the ability to upgrade appeared.
The transition from two to three is smoother (essentially, only specialization and a second hero class were added).
The fourth part is somewhat similar to the second (specialization was removed, heroes of the same class became similar). Although the fact that heroes started fighting is an innovation.
The fifth part = Heroes-heroic spirit. Taking three as a basis (specializations, upgrades for all), they added the progression system from 4. At the same time, they completely killed the unity of the MM universe by slapping together a painfully banal story about evil Necromancers and demons wanting to conquer the world.
I most often play two and three. Right now, I'm playing four. I haven't touched five since the last Tournament.