1, 2 - haven't played them, but honestly, I don't expect anything good.
3 - a good game, my favorite after part 5.
3.5 - incredibly unbalanced, but it's still fun to play with this feature. It's enjoyable for a while, but then it gets boring.
4 - I played it a little, but I didn't really like it. It doesn't resemble Heroes very much.
5 - my favorite part. And it's not just about the graphics. The development system is better than in part 3. There are a lot of interesting abilities, and each faction has unique features. And there are many other small things that make it better than part 3.