Demons have one feature that many rarely consider; with properly developed ballistics, Jaleh has a huge advantage, all other things being equal.

There is a certain drawback, of course, but there is also a significant advantage if the ballista is not killed in time and Jaleh is a high level. I also liked this hero, although even without him, there are plenty of others to play with; in this case, the size of the map itself plays a role - this is a fact. And the fact that the ballista is resisted is, of course, a drawback... but three fireballs (more or less) with decent damage, and if you also add luck... mmm... they can do a lot of nasty things.

Not so long ago, I started playing Heroes 5 out of boredom, I think I'll find an opponent and try to play online, because before that I was playing against bots, which is a bit sad. Although, sometimes, bots can be incredibly strong, compared to real players. This has happened to me many times on random maps, especially in Heroes 3. In this case, there are also factors that depend on chance. :(
Each race is strong in its own way; of course, the best one will be the one you play best. Personally, I replayed the same map with different races, one after another, to master them, and each has its own starting advantages in development, which subsequently affects the mastery of the map and further battles. We develop faster, the army grows faster, the hero grows faster, etc.
I would prefer three factions: Humans, Inferno, and League of Shadows. This is simply due to the effectiveness of individual components of these factions. Of course, Necropolis is strong, but playing with a strong demon army with high initiative and speed, even with poor stats, I managed to defeat armies that were much stronger than mine. Of course, the bot is not a human and could have acted smarter, but, again, let's go back to what I wrote above: race + skill + tactics combined will only give a result.
If, however, we need to choose the strongest faction, I would give first place to Necropolis; they are very powerful, especially with a good strategy, and if you also
play on a large map, it is very difficult to win against them. Huge hordes of quite strong undead can crush anyone. The fact is, you send a strong hero on a quest, and you can forget about supplying the army; the only BUT is the availability of dark energy, which is not always enough. :(
P.S.
If anyone wants to play on a small map, please write to me in ICQ or here in PM :) I'll try, although I currently have version 3.1, and there will be some questions regarding installation.