What unit should I upgrade?

One of the merge projects, so either Hana or Soren.

Soren or Hana, since merge projects should be your highest priority

+Def -Spd Tiki, if you don't already have a 5* Tiki. She is one of the best units in the game. Give her Aether from a Chrom eventually.

Those are all good choices except for -Spd Raven. And if Soren is going to be the base for a merge project, you might want to hold out unit you find a -HP or -Def Soren. Res is a superboon and you probably want to keep that intact either way.

Ok, people are assuming you have merge projects, and if you do then prioritize them. Just because you said you could merge into +atk -res Hana, I'm not assuming that you intend to max-merge her.

If you don't do merge projects, pick the unit that interests you most or the one most needed in your roster right now.

+spd -res Raven. Awesome unit (great refine, nice art).
+Def -spd Atiki. This unit is really uselful, mix damage, great defense... Yes
Valter. This guy helps me a lot in some maps. One of few blue unit with some bulk. And you can think of great builds with him (cursed lance is underwhelmed)