Sort by Obtained. My oldest is Golbez. Pretty sure I got him on his banner. I had myself convinced he was a complete badass, and used him over Exdeath until Exdeath got his 6 star Awakening.

Bonus question. Who is your oldest 5 star base? Mine is Luneth, again, on banner, and used him pretty much all the way until I picked up Ang.

Extra Bonus. Who is your oldest 5 star base without a dupe? For me, answer is same as above, Luneth. I have three Aileens, and tons of other dupes, but not for my oldest 5 star. Olive was my 2nd, and no dupe for her either.

Edit. I purposefully phrased it the way I did. I think most people here will have Magitek Armor Terra, but when did you awaken her, and thus make her a new unit? Or did you even awaken her in the first place? I thought some people would be surprised to see who is their oldest unit that hasn't been awakened, or has been capped, as is my case. My Golbez is lvl 80.