Who to 5*?

Michalis for the win. It was the first and last unit from a GHB that I level up to 5 stars.

If you choose just one, M Robin i fully recommend. I find him very useful with a Raven tome + triangle adept + sword/bowbreaker build -works really well and he’s been very useful to me when completing various GHBs and such.

Eirika is also great as a buffer, but I recommend you wait and see if you get a +speed one. I have her at 5*+1 with a speed boon and she’s great. She’s fairly common as well, I currently have 5 x 4* copies of her.

Well I can already tell you that Olivia, Eliwood, F!Robin, and Navarre are all ones you shouldn't consider 5*ing for usage purposes, only do it if you like the characters or somethin and make them work like so because they aren't all that great.
Cherche is amazing if you have access to a brave axe and death blow and same goes for Chrom.
M!Corrin works great if you have a swift sparrow to slap on him and he's +atk.
M!Robin can work great as a 40+10, but 5*ing him wouldn't hurt.
F!Corrin only if you plan to invest a lot into a dragon team.
Cecilia is wonderful if you have a spare Nino to throw at her for Gronnblade and you're putting her on a horse team.
Michalis is kinda like a defensive Cherche, just make sure you have a QR on hand.
Eirka is always a nice choice because of her supportive role she has to boosting stats.

It depends on IVs. Most of them are better at 4* +10, although I personally hate that silver boarder so I’ve never done that before.
Now assuming you are like me or you’re somewhere in between, I’d say the most worthwhile units would be; Cherche, Chrom, Robin(M), Robin(F), Cecilia, and Michalis with a shoutout to Olivia (I would definitely add Eirika if she has good IVs, but yours has pretty average ones). The others are really only worthwhile if you like them as characters or as units (in Eirika’s case, if you really need to use her) whilst the ones listed above are very reliable and relevant in the meta.

Personally, I would just upgrade the one you like the most with the best IVs.

It depends on whether you actually need that unit to fullfill a specific role or whether you just want a fun character to level up and build. Out of everyone that you listed, I had the most fun playing with Eliwood. He does get seriously outclassed by other red horses, so he's by no means top tier, but I love his gimmick which he gains with a +atk IV and that is one-shotting people thanks to durandal + death blow 3.

Of the ones you've listed, I'd go Cecilia.

She can very competently run both the Blade tome build (and she only needs regular Gronblade given cav buffs - you don't have to sack a 5* nino) and obviously the raven build. She's a slower unit and you'll probably want to use ward/goad with the raven tome, so getting the + damage is much more important.

I built her as a 4*+10 (sacking one 5* in the process to give her Raven+) and she has been a staple of both PvP and PvE content for me. Since a 5* is roughly equal to 4*+6, it's worth doing unless you plan to build a full 4* merge over time.

Of the rest, Chrom, F~Corrin, Eliwood, F~Robin, Michalis and Eirika all benefit appreciably from being 5* (primarily thanks to their weapons at that level) and are all useful depending on the rest of your available units.

Olivia and Cherche should be built as 4* merges since they are plentiful and do not gain anything useful from being promoted to 5*.

Cecilia or Cherche are the ones i’ve had the most fun with.