A difficult question. There isn't really a universally strong monster here. And while you could consider the Ballista the best ranged unit, it severely slows down the army on the march.

That's absolutely true. Level 2, in my opinion, is the most balanced in the game; on the others, there's always someone very weak and someone very strong (like Ogres and Vampires on level 3), but on level 2, everyone can be the best in a certain situation. But I still think the Ballista is a versatile unit – its ability to shoot without penalty through walls is unmatched, and almost all other monsters have their own "counter-abilities." I agree that Ballistas are unfortunately very slow, but in Haven, you can build a stable, which partially compensates for this disadvantage.