I think the easiest Hero to add using abilities in the game right now would be the Ranger, a super Scout/Elf.
Anti-Air Leadership - This game is now vomiting powerful air units left and right, especially Demons. Pair elves, wizards or eagles with a high-level Ranger and you can take down air units with ease.
Ability to get to lead armies through any terrain with 2 movement (maybe start off with 4)
Ability to increase group move more than the other heroes.
Ability to increase personal move higher than the other heroes.
Starts off with a higher sight radius/ability to increase sight radius higher than the other heroes.
You probably wouldn't hire this hero at the beginning of a game, but I could see him being very useful to get midway.