Those who don't play Heroes at all hate Heroes 4, but it's simply less popular (relatively speaking).
I would rank them like this:
1. Heroes 3
2. Heroes 5
3. Heroes 4
4. Heroes 2
5. Heroes 1
And that's solely based on the fact that Heroes 3 was a huge hit, and the rest follow in descending order (the newer, the more popular).

But we're not talking about that. We're talking about neutral creatures.

Phoenix is a strong unit, but the rest are...

That's not true... I started playing Heroes with the first game, and the fourth part just drove me crazy... it's, in my opinion, a complete mess. And the fifth one, in my opinion, is better than the third... at least there's a plot.

And the neutral creatures are very strong. The Earth Elemental is quite something... it has amazing stats and a constant attack, you can't take it down with just cannon fodder. And the Water Elemental is strong too. I don't remember the rest... And the Dark Knight is decent, although its appearance... well...