Some of the Leader cards are more powerful than others.
Morphun, Bridget, Hugo and Timber lead the pack in the best. If I had to say hands down the best one is Morphun. 3 more Pillz per game makes a huge difference.
Vansaar is great for leveling characters.
Ambre, well we can't seem to determine if her ability actually does anything.
Ashigaru is very good but can open you up to a turn 1 kill, and a deck has to be carefully planned around him.