5 star upgrade

Easiest to build is Tailtiu. Best speed, have bladetome+.
I would give her Darting blow, Desperation and she is good to go (with some buffers in her team). IV could be better but at least she is not - spd or -atk.

Robin can deal with archers better with more attack and defense and has good IVs for it but if you want a general, hard hitting blue mage, go for Tailtiu. The issue with her is her IVs aren't the best and I wouldn't advise spending 20,000 feathers on anyone with anything less than perfect IVs unless you're going to be promoting another and merge them later. I'd advise Robin and save up for and hope to summon a better Tailtiu or be surprised by Linde or Delthea.

There is many definitions of a good blue mage. It all come down to what you need.

What are you looking for ?

Robin is a raven tome user and can do a decent job with not too much investment (can work even with TA2).

Tailtiu is the one with the more firepower of the 3 and can run a decent blade tome if buffed by her team.

Mae is a sort of hybrid thing that relly a lot on positionning to be efficient, but can shine well with appropriate support. She have a decent res and depending on her IVs, and your choice of upgrade can be a pretty good mage tank.

Like most of the guys below have said its a matter of what u want from your blue mage. Robin is great if u want to take down Clear units like archers and dagger units (recently healers are a monstrosity to try to bait so yeah, dont try it)

Mae is an all rounder and if u run her base tome (owl tome) she could be good if given the proper support along with spur buffs, almost like an armored mage. She's good and her IV's are good as well, one of the better options.

Tailtiu however is where things get kinda weird. She is the pure offense character out of these, and has no chill when it comes to damage overall. I do run LnD3 on her, but with darting blow and Blade tome buffs she hits pretty hard.

Overall, if it was just for the utility in current Meta, like what they kill and how it helps. It would be Rob, followed by Tailtiu and Mae sitting tied. Rob in the current Meta can kill BraveLyn (very important for arena) and can easily kill other units like Black Knight and Zelgius with some support.

If this is just for how "powerful" they are, Tailtiu sits uptop as the only full offense mage here, and Mae sits in the bait category, since owl tomes require some really good positioning. Rob sits in the back since his power depends on who or what you're aiming to fight which has made him kinda niche since day one (had rob since day 1, have been sad to see him do so little recently)