My favorite is H3 with WoG. Right now I am writing CARAVANS script for H3.
We can emulate a lot of things with ERM. If You say, that H1,2,4, or 5 is better, because it has some new options, we can write them for H3.
I also plan to write a Gating spell, amongst others..
It also has Commanders- they are something like H4 heroes.
And you can choose, wich game rules you want. Everybody can play with his own game. This modularity grants 1st place.
After that comes H2 for me. It is similar to H3, has a really good feel- just everything on the right place. Do you know about H2 mods? I don't.
H5 could be better, but the 3D adventure map with camera handling and not visible treasures go on my nerves. If you could create a mod with 2D adventure map (like H3's) plus 3D battlefields.. PLEASE!!
The usage of the skill tree could be ok, but not with random skill advancement.
I prefer hexes over cubic battlefield. (ok, it's my personal social problem
I also have a not supported GF4MX440 card, so I don't see those nice animations, just ugly textures. It also counts. I have heard, that the mapeditor is also complicated.
H4 had a lot of fresh ideas. Unfortunately they have ran out of development time, and released a beta version. After that, they didn't had the chance to introduce major changes. I have played only without the expansions.
Did You noticed, that we have 5 different skills, just to add mana points?
In H3 we had intelligence. Period. It was also problematic, that hero class changes with the last skill advencement. You may change your class at every levelup.. Brr...
H1. It's nice, but I don't waste my time on it. I have something better.
Biggest problem is the magic system without mana points.
King's Bounty: I haven't tested.