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Wing is definitely number 1! It's a bit tough at the start, but his ability is noticeable even at level 10, and at level 15, he becomes a beast!
Beautiful Ilfina, just like in life, has only a beautiful appearance, but inside... With 1 horse, you can only take stone and wood; the rest of the mines are a no-go!
Ossir is very easy to take, but the fact that he loses himself when the archers are destroyed (and any sensible opponent eliminates them first in Sylvan) is a drawback! But the start is fast!
In my opinion, Wing, and only Wing! He eliminates at least 1 unit at the beginning of the battle, and if he reaches a couple of melee units, then victory is already in the bag!
I'm not considering the others because I don't see them as contenders!
P.S.:
All of the above is based on the "hero" difficulty level.