My favorite is the Anti-Magic diploma.
+2 Defense
+2 Intellect
+25% Magic resistance (absolute)
The magic resistance bonus is really, really nice. You will encounter a lot of heavy stacks on impossible full of units that inflict magic damage. That one saves butts big time
The Botanic diploma:
+10 attack and defense to all Thorns, Ents and Dryads
Generally it's not bad, because 10 attack and defense is a very serious bonus.