Path of Warrior's Lore : Poison Damage, Fire Damage, Lightning Damage
Path of the Warrior Gods : Spiritual Damage
In the Warrior's Lore section I also didn't bother with Repel Damage (in general I don't like these types of skills, even in games where the reflected damage can be much greater than the original taken), but in Warrior Gods, one level of Reflect Missiles came in handy (I found it helped me locate archers just a little bit quicker, especially in areas with lots of cover, like the Dark Forest).
Take a look at the skill descriptions and decide for yourself which ones to try (I have not played the download version yet, and even with the disk version different people like different skills/spells). You can always save, go through an area of opponents, then reload, learn a skill, and try again to see if you notice a difference (though some skills may need to have 4 or 5 skill points in them before you are likely to see a dramatic difference in a small test area).
Based on the description I never bothered with Stun until I happened to find a sword with level 2 or 3 Stun on it; it helped more than I thought it would, so when I eventually upgraded I started investing in the passive Stun skill.
I used several swords with versions of the passive damage skills on them. The extra bit of damage was nice, when it got triggered, but IMO not enough to invest my own skill points.
