What is your favorite ultimate ability for that class?

Don't overestimate the capabilities of the Watchtower. Even a triple Watchtower. A stack under the protection of a Watchtower can be easily destroyed with focused attacks, forcing the hero to spend their actions on repositioning it.

Indeed, it's not worth it. :)
But still, if you apply it to Paladins, the effect will be very good, and the Knight doesn't really need to move troops forward... he can shoot from a distance. It's very effective against undead, which only have Vampires, Dragons, and Ghosts as fast units (and even then, relatively). You kill one stack... and place the Watchtower on another. By the time they destroy a couple of stacks, they will have lost half their army...
And the skills for Strength are useful.