HOMM3 is best in the series of cause
1) fans made some tools to extend it's engine
2) graphics is better than HMM1/2 yet don't require 3d acceleration and dont have ugly 3d units like 5-6 has
3) it's easy to create graphical content for it
I personally liked Necro most.
Than Castle and Inferno (althrough Inferno is weak in HMM3).
In HMM4 i didn't like thing they did with heroes. And graphics was too small and too 3d-rendered.
But idea of monsters moving without heroes (and guarding monsters patrolling too) is good, it's good if it will emerge in VCMI as option.