Consider the following. A level 18 wizard. For the lvl 1 spell of Spell Mastery, they chose Shield. Whenever they get hit by an attack roll, they cast shield and add 5 to their AC. This basically trades in the wizards reaction for +5 AC. The one counter to this is saves, but there's probably something you can do with the level 2 spell.

I have shield and misty step for my al chacrter

This is a pretty normal use of their 18th level feature, and by that point it isn't terribly unbalanced compared to their normal schtick. 

Still, take one level in fighter, and your AC is through the roof

Which is more useful? Absorb Elements or Shield? Or does it vary depending on the campaign?

At early levels? Shield I feel is better just because you fight less spellcasters. Once you start reaching tier 2 though taking both is a good idea just so you are ready for either common situation.

Quote from Tim >>

Which is more useful? Absorb Elements or Shield? Or does it vary depending on the campaign?

 In general or for Spell Mastery?  Really depends on the game but Shield overall has more things it applies to.

What are some Dunamancy spells that would be worth it?

For Spell Mastery?

Fortune's Favor and Gift of Alacrity.  Having everyone in the party get an effectively permanent +1d8 init and always start battle with a free D20 roll that they can use to replace any other roll they make is pretty powerful.

For my wizards of high merit, I never chose shield, because if I need a +5 to AC to save me, then the rest of the party is dead, and I need to leave with some teleportation. However, I usually chose Fog Cloud, and Identify. Heavy obscuration is almost a requirement for such high levels. If youre fighting a high level archer, fear not, Fog Cloud will cover you up, and you can cast it at will. Identify does as it says: It identifies magic items and properties. This is useful for all levels, especially since you recieve many more magic items at higher level than at lower levels.

Not only can Identify be used with magic items, but anything else magic! There's a pedestal with a bowl in the middle of an Evil Elemental Air Temple? Identify, and it turns out that the bowl is a Censer of Controlling Air Elementals! The limits are endless when at such high levels, almost anything can be magic, and you can identify it at will.

Identify is a freaking ritual.  You really don't need to be worrying about spell slots with it.

Shield is useful, but not so much if you're a War Magic Wizard.

Quote from 6thLyranGuard >>

Identify is a freaking ritual.  You really don't need to be worrying about spell slots with it.

 Well, I dont wanna spend 10 minutes casting it if I can do it at will. But hey, I base my wizards mostly on utility.

There are a lot of other 1st level utility spells, though.  Most of which have more immediate uses.  Like Feather Fall, Silent Image, or Charm Person.

Quote from 6thLyranGuard >>

There are a lot of other 1st level utility spells, though.  Most of which have more immediate uses.  Like Feather Fall, Silent Image, or Charm Person.

 I see your point, however, there may be party members who can do that for you. I think thats where we differ. I play based on team building, and you might optimize a single character for high damage output and/or utility. This is not a criticism! Just an observation!

I would just use identify as a ritual.   Shield and misty step are usually the defaults unless there's a reason to change them out.

Quote from 6thLyranGuard >>

There are a lot of other 1st level utility spells, though.  Most of which have more immediate uses.  Like Feather Fall, Silent Image, or Charm Person.

• Feather Fall -- decent enough but for how often it comes into play prepping it as opposed to mastering it seems better
• Silent Image -- nothing scream power like replicating a 2nd level warlock ability.  :P
• Charm Person -- still a bit situational but not bad.  I'd more likely use the friends cantrip because people know they were charmed and abusing that can have long term negative repercussions.

Grease for fun.

Quote from Ashrym >>

I would just use identify as a ritual.   Shield and misty step are usually the defaults unless there's a reason to change them out.

 

 

Quote from 6thLyranGuard >>

There are a lot of other 1st level utility spells, though.  Most of which have more immediate uses.  Like Feather Fall, Silent Image, or Charm Person.

 

• Feather Fall -- decent enough but for how often it comes into play prepping it as opposed to mastering it seems better
• Silent Image -- nothing scream power like replicating a 2nd level warlock ability.  :P
• Charm Person -- still a bit situational but not bad.  I'd more likely use the friends cantrip because people know they were charmed and abusing that can have long term negative repercussions.

Grease for fun.

 Im going to be completely honest, I totally forgot that there are 2nd level spells for spell mastery, and I would, without hesitation, choose Misty step. Its so useful in like... Every situation... Unless youre talking to your ex... Nah, even then!

Don't underestimate the benefits of the Blur spell.

Quote from Dertinus >>

Don't underestimate the benefits of the Blur spell.

 The problem with blur is it is concentration.

Mirror image is way better, and is second level.

I don't usually bother with blur or mirror image for mastery.  Blur is only better than mirror image if a person is expecting to need to use it against a lot of attacks over time and capable of maintaining that concentration where mirror image gives better protection initially then hits that hard limit from the number of images available and also doesn't require concentration.  In both cases I can just use shield at will at that point and cover a lot of attacks and not use concentration, and not lose my 1st round attack casting a defensive spell.