http://samanna.net/wiki/index.php/Paladin_AAs_%26_DiscsTanking- Combat Stability and Agility to at least 8, first and foremost. 27aa for 15% avoidance, and then another 27aa for 15% mitigation.
Maxing CS and CA should always be high priority, but I can understand wanting to mix things up a bit.
- Shield Block to at least level 3: 5% more shield blocking for 18aa.
- Natural Durability 3 is 6aa for 5% more base hps.
HealingHealing Adept and Healing Gift increase the power of the Ward of Tunare line dramatically (not to mention burst heals, group heals, etc). Healing Boon does its part as well, but I don't think those AAs are necessary until much later on.
- Healing Adept 4 gives 13% more heals for 15aa
- Healing Gift 5 gives 14% more heal crits for 17aa
- Hand of Piety 1 for only 3aa. Free 1125hp heal. It's affected by Healing Adept too. Later levels are awesome, but getting tanking AAs are more important early on. Max Hand of Piety does 9600hp or 12288hp with max Adept.
DPS & Utility- Innate Run Speed 5 is really nice for places where you can't use the aa horse.
- Veteran's Wrath 1 is only 3aa and increases your melee crit damage from 1.7x to about 1.9x your max non-crit.
- I'm not sure about the level requirements, but getting the highest level of Hand or Force of Disruption should be pretty high on the get-list. It's extremely convenient to have a mana free aggro spell not only to save mana during fights but also so that you can still aggro after a battle rez or a mana drain. Hand of Disruption has 1 second cast time for 1000 hate. Force of Disruption 1 is .5 second cast time for 2000 hate and Force 2 is 3000 hate.
If you want some round about figures for certain AA levels, I still have an old document I made some years ago. Some info is more than likely incorrect, but it gives you a general idea what you're getting out of your AAs.
http://jeffmania.com/uploads/aas.html