Who should I upgrade?

Vault

Str

4* GR to rank 5

Depends on gaps in your roster.

For utility, gr is the best option. Powercontrol, bleed immune, regen, works well with mystic dispersion, but low damage.
Or

For damage, hyperion is the best option. His regen is alright and damage is high. Also poison immunity is fairly useful.

cx234333

colossus

shadow_lurker22

5* Hyperion to rank 3

Do the 5*

rockykoston

5* Hyperion to rank 3

I just upgraded my 5* Hyperion to Rank 3 and he literally demolishes opponents where I used to struggle a bit.

Spam SP1 and see the incinerate damage numbers fly, or the hard hitting SP2. I am only sad that I have nothing for his synergy.

Vault

Thanks, I have just rank up my hyperion.