Strange, Heroes Of Might and Magic III is still the most popular one of the series.
It's like Final Fantasy 7, it's the first one many played. Doesn't make it the best, but the one most people discovered the series with (it's just nostalgia).
I don't know all communities, but the only ones I see still active (with games played all the time) on France oldest homm site are h4 and h6. H3 community stopped playing long before h5 came out.
Some upgrades are essential, like with Vampires or Gremlins.
You mean some creatures are just useless unless upgraded. And others don't get anything from being upgraded, beside some stat increase, which is just artificial upgrade. No new skill, no new purpose, just the same but somewhat stronger, so why bother making it 2 creatures ? H5, with ToE is in that perspective the best in my opinion, as it allows for alternative upgrades, some of them having different purpose (other being useless or weaker).