Saruman is an amazing leader.
His Epic Channeling and Epic Ruination abilities make him one of the best mages in the game.
The Voice of Saruman ability is also very useful to keep his company out of combat, and to avoid being killed in a duel.
He is Touched By Destiny, which gives you the extra might needed to make sure that his action succeed.
Lurtz is also a very good character, but nowhere near as good as Saruman.
His Assassin's shot and Mighty Blow abilties are very good against cavalry and Ents.
Thrydan Wolfsbane is an inexpensive hero for small games.
He is particularly effective against the forces of Rohan.
Mauhur's Maurauders are my favorite Legendary Formation of Isengard.
They are cheaper than Uruk-hai Scouts with Shields and the Man-Flesh special rule is devastating.
Ugluk's Raiders are also very good.
Their Furious Charge special rule is very useful, espically against cavalry.
I am not a fan of Vrashku's Talons or Sharku's Hunters.
Although shooting is very powerful, Vrashku's Take Aim special ability costs a point of might and only adds a few shots.
Sharku's Hunters are more expensive than Warg Riders.
For this they gain Expert Riders and Savage Onslaught.
The latter would be useful if facing low courage enemies, but most good armies have high courage.
Expert Riders is useful, but only adds a few shots.