The Tavern

For a BaIE of Vehumet with Ozocubu's Armour, Ozocubu's Refrigeration, Freezing Cloud and Absolute Zero, which armor would you choose?
(Lets say I have no other resistances from items, just a staff of cold.)

Robe of night due to the power of the darkness passive. +5 steam dragon armor is quite powerful as well with Ozo's.

Yes, I would take the Robe of Night, which would bring Zero to LOS for the Barachi.

robe of night is the best body armour item in the game

If you have no other source of MR+, I would go with Robe of Night.
If you have MR+, but don't have any of rF+/rC+, then robe of resistance (though I, probably, wouldn't use it for rF+ only).
Reducing LoS is less important for blaster caster, IMO, and having no resistances and reduced LoS on a slow species would mean that sometimes enemies will enter your LoS and instantly blast you for huge damage, and there's no real reason to take that risk, especially seeing as you have refrigeration and AZ already (as an aside, regarding refrigeration, be super cautious about using it in Z:5, being exploration teleported into lungs with -Potion would suck badly).