What is your favorite ultimate ability for that class?

I disagree with Lord Vampire...
True luck... I don't think the skills needed for it will be useless. You would have acquired them anyway, and here you also get a Special Ability.
1. Native Lands... +2 to speed on grass:)
2. Storm Wind... -1 speed to enemy flying units:)
3. Secrets of Light... wizards in the army spend 50% less mana when casting light spells. Dryads can dispel spells twice:)
4. Elven Luck - no comment.
+ Overall speed increase, overall retaliatory strike
The Special Ability of the Sylvan elves is quite easy to acquire, unlike others.
It's also very easy to acquire Unrelenting Force, especially with Laslo and Mive:)