Racial abilities

Haven and Necropolis are strong, and their faction abilities are useful. Necropolis does have only a resurrection ability, but it's an effective one :). Haven also has a resurrection ability at level IV – perfect for a decisive battle, and before that, the Guardian Angel is an interesting tactical ability.

Opening portals only works in the later stages of the game, and only if you have a lot of troops... Honor and Fury strengthen the army regardless of the number of units, but Fury's first level ability is also effective, and Honor reveals its potential at level four (based on my observations).

Overall, I also vote for the Guardian Angel :). In my opinion, it's the most unique ability.