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Yes, Ossir is definitely good, especially at the beginning, as the archers give an advantage, and the luck bonus in the starting set is also helpful. But in the middle of the game, the importance of Vinrael's ability starts to show – this is truly a wonderful ability, because imagine, at level 15, he will gain 30% more experience; isn't that great? In my opinion, this is my choice when fighting for Sylvan. Anven is also not bad; he's a good hero, although he depends heavily on luck (what monsters appear on the map). Wingael can also be useful; dancers and nymphs can always use an extra row, and even more so to hide the archers and druids.
The other heroes, in my opinion, don't quite measure up to the ones mentioned above...
P.S. And isn't Anven just a copy of the elf from The Lord of the Rings? There's even a typo in her biography where her name is spelled Arwen.