The following is my Cassandra build. She acts as an almost fully autonomous tank - the only time I give her directions are the few times I use the tactical mode, where I select her to give her the target to focus on and then leave her alone.
I started out giving her all sorts of defensive abilities, only to notice that this wasn't needed at all. This is on hard mode and she is tanking for a rogue and two mages. I guess that more babysitting and a greater focus on defensive abilities would be needed on nightmare.
The following is her build by level 18. The core around which she has been built from the start is Challenge and War Cry from Vanguard and Payback Strike and Lunge and Slash from Weapon and Shield (with upgrades), followed up Spell Purge, Blessed Blades, and Rally as soon as she got her Templar specialization.
Her abilities on the hotbar are:
- Challenge
- War Cry
- Payback Strike
- Lunge and Slash
- Spell Purge
- Wrath of Heaven
- Blessed Blades
- Rally
All except Wrath of Heaven are upgraded, and that will get upgraded on next level up. She has Shield Wall that isn't on the hotbar that I keep in reserve if I ever need to switch it in for a boss fight, but as I found that the AI didn't use it a lot (or at all) and she had no problems surviving without it, it didn't justify being available for general use. I haven't needed it yet, but as I have not finished it yet, I guess there could be some boss in the few remaining areas it would be useful for. And I need it for access to Bear Mauls Wolves anyhow.
Rally is my "I hope I never have to use this, but if the **** hits the fan, Rally will give me time to get a battle under control" button. (In practice Rally ends up only being used in a few boss fights, but I'm fine with that. Better safe than sorry.)
AI Cassandra keeps up Blessed Blades, and while she's frequently the only one in range, it does mean that her Spell Purges are frequently available in fights where the AI sees a reason to use it.
Going with Wrath of Heaven rather than Shield Bash was hard - in most fights AI Cassandra gets good use out of Shield Bash, while Wrath of Heaven is seldom used - but she does use it in large fights against demons, which is where it matters the most. I'm still not quite sure about that choice from a pure min-max perspective, but in the end, she is a Seeker with Templar abilities. How could I justify NOT giving her Wrath of Heaven? 
I have all the tactics enabled save Rally - there doesn't seem to be any need to set any to preferred.
Her passives are:
W&S: Bear Mauls Wolves, Turn the Bolt, Turn the Blade
Vanguard: Trust the Steel, Cutting Words
Templar: Maker's Will, There is no Darkness
Plans for development over the last few levels/power medallions in the game:
Templar: Wrath of Heaven (upgrade)
Templar: Champions of the Just (passive)
Vanguard: Untouchable Defense (passive)
Vanguard: It Will Cost You (passive)
...it is damn tempting to instead go for the three points invested in Battlemaster to pick up Deep Reserves, but let's face it - at this high level with all necessities covered, it is really a choice of which delicacy one prefers.
Her equipment is:
Best sword I can craft
Best plate I can craft
-- one of the two always has the "on hit gain 3 guard". So innocent looking that text, so great an effect.
Best hat I can craft
Shield with "heal 15% over 10s" from the moment I picked up the Wedge of Destiny.
--Note that this isn't as great as it sounds, as only the last source of damage is counted rather than stacking, but it is useful, particularly for an AI tank that is controlling its own potion use.
Enhanced Ring of Challenge
Enhanced Ring of War Cry
Best Stamina Amulet I've got
Best Guard Belt I've got