Like I said in SIN's thread, (consistent) feats relative to others, and powerset. Monarch has both IMO. Wrecked roomfuls of GLs/Firestorms/Supermen, and dropped all 48 versions of Captain Atom before absorbing their collective energies. Doesn't hurt that he's got the power to destroy universes (when "completed"😉. Working backwards, I damn sure wouldn't say that they are any HHs or trans tier characters who could replicate what Monarch did.
I'd put him comfortably in the skyfather tier. Never really given much thought to how high, though.
I would place Monarch pretty high up there as well but I would say around high trans. Monarch really didn't do anything to say he wouldn't get stomped by Odin (unless his suit is ruptured). Monarch is missing versatility. Prime only good showing against Monarch was him ripping his suit. Before this, Monarch was slowly blasting the GA out of him. I didn't see anything in that fight that made Prime equal to or on Monarch level.