I’m assuming specifically anima/elemental mages rather than anyone in a light- or dark-based magic class.
As units:
1. Ced – yeah, FE4 holy weapons are ridiculously broken and I did indeed go with Lewyn!Ced for my only full playthrough of the game, so this one was no contest.
2. Pent – like many first-time players Pent left a big impression on me in the desert map where he’s an NPC stealing your kills, but I can forgive him for it since he comes ready-made to be your most valuable caster with his good bases, high tome and staff ranks, and auto-A support with a decent filler sniper.
3. FE11 Wendell – why is an overleveled unit you recruit so early so good? Particularly with the DS games’ minimal level of aesthetic and characterization differences between units Wendell just makes leveling a whole bunch of casters and potential casters kind of pointless.
As characters:
1. Bastian – both amusing and unexpectedly clever when the plot comes knocking, it’s a shame he’s not around more…and that casters are barely worth using in Tellius. I imagine translating him across multiple localizations must have taken some work.
2. Tailtyu Tine Arthur all the Freeges – whatever, they’re all mages and all screwed up in interesting ways, they can share a spot. Too bad that most of their dysfunction happens offscreen, but narrative and console limitations are what they are.
3. Orochi – a fortune-teller who trolls people in usually hilarious ways, and she’s one of those characters who outline Hoshidan spirituality which is always interesting especially when it’s irreverent.