Diplomacy - I take it, but not always.

Yeah. Considering:

Pathfinding - very useful skills, I always try to take them.

and:

Mysticism

Tactics - IMHO, why? Why arrange troops? We wait until the enemy army makes its move and then crush everyone as needed.

- tactics are a very good skill (I can explain).
And calling armor weak is wrong - it's better than attack.

Added after 13 minutes
As for the spells: I try to take them all. This is the best. There are others, but these 4 are essential.
(IMHO, the ideal set of spells: level 1: Haste, Slow, Bless, Shield, Curse, View Air, possibly Mass Dispel and Healing; level 2: Blindness, possibly Accuracy and Lightning; level 3: Amnesia, Teleport, Animate Dead, possibly Force Field and Earthquake: level 4: Berserk, Clone, Town Portal; level 5: Dimension Door, Fly, Sacrifice.