What would Unicron from Transformers be?

I think he would be at least an Old One since he destroys whole planets like Galactus and he is as big as a planet.

IIRC Unicron is supposed to exist on multiple parallel universes at once which would suggest he's at least a First One (multiple planes associated with one dimension) or possibly an Eternal (multiple dimensions comprising one Reality).

Neutronium Golems are Eternal-built constructs and they are described as being able to shatter planets with a punch... but despite their incredibly high CR they are not really Eternal level themselves, they lack the 'absolute' abilities (the Dimensional Masteries from the First One/Demiurge/Time Lord templates, Transversal, Perfect Defense, etc.) that a real Eternal would have.

Unicron has an 'equal and opposite', Primus, in the same "Reality" so I would suggest that he is probably a First One rather than an Eternal.

In terms of sheer power I'd say an Old One or Elder Deity, in terms of fitting into a cosmology or campaign pantheon or Multiverse, I agree, First One.