Hoarding ALL the divine dew

Lucina, Hinoka and Minerva are the only ones I've done from your list and they work really well with their effect refinements. Lucina can make excellent use of Sealed Falchion or Spectrum Bond. Minerva loves the +10 damage added to Moonbow or Ignis. I rarely used Hinoka with her Brave Lance but the upped stats from her spear and the improved movement for her team really changed her to a very usable unit for me. All improved noticeably so I'd definitely recommend them.

I've been fairly liberal with stones and dew until recently when I've started saving for no particular reason. It's skill I have troubling foddering off in case a new unit could use them better, I've got 2 Galeforce, 2 Bold Fighter, 1 Wrath, 2 Swift Sparrow, 6 Distant Defense to name a few sitting around waiting just in case.

It really depends on the units you‘d like to use. But I‘d say Leo, Jeorge and especially Jaffar aren‘t really worth it. Leo and Jeorge can already be very effective with a Bladetome and a Brave Bow and Jaffar is outclassed by healers when it comes to chip damage.

Other than that it really comes down to your preferences and team composition. And if you want to use them on your main arena team, avoid spending your Dew on 5* exclusives or ghb units, since they are very hard to merge.

If I were you I‘d refine Alm‘s Falchion for Arena Assault and PvE content and spend the rest on a non 5* exclusive unit you‘d like to merge to +10.

The only Divine Dew I‘ve spent until now was on Raven‘s Basilikos and he‘s insanely powerful and nearly at +10 merges. I would‘ve spent some Dew on my Alm, but since he‘s +Hp /-Atk I don‘t think it‘s really worth it.

My 'most useless' refine probably was Seliph's, since with so many red swords and with his terrible IVs I use him very sparsely. Its not a bad refine, but a waste if you dont use the unit

The units that I have refined from your list are Corrin, Caeda, MLucina, Seliph and Raven.
From these ones I would say that Raven's is the most powerfull one, then Lucina (since I use her a lot on TT as a passive healer), Caeda, Corrin and Seliph respectively. Thats how it works for me at least with their IV's, merges etc

Now, Michalis and Roy have great refines, and if you dont use Hinoka that much her refine might change that.

I dont recomend refining Eldigan's weapon now if you dont use him that much and/or dont have good fodder for his A slot. Double fury kills him and he will probably get Dark Mystletainn in a few months just like with the awakening falchions. I had been meaning to upgrade his sword but seeing Ares made me want to hold onto the dew for a while longer

seliph

I have a hard time deciding what to use my dew on as well...

I've upgraded the following:
Lyn - I'm not sure it's worth it but I just wanted to make her as strong as possible
Ephraim - Totally worth it. Love the double attack.
Marth - I like units who buff their allies, so I found this to be a fantastic upgrade. With refined Falchion and a drive C skill and seal he gives +2/+2/+5/+5 to all units within two spaces during combat.
Jaffar - I really like his chip damage in PvE maps. I thought this one was worth the upgrade
Hinoka - Questionable. I use her on my flyer team so I upgraded. It's the type of skill that doesn't get used much imo.
Alm - Everyone says how great it is...I guess it's good. Just haven't used him much.

Of the units you mentioned, Minerva seems like she could be a good choice. If you want to pair Corrin with someone I feel like his upgrade could be great.