Well, if they're both at peak, I'd take Mephisto.
I'm assuming since he's at 'peak' he'd be able to use his powers full bore regardless of location, in which case he'd eventually overwhelm Darkseid. If it took place in the M's Realm it would be a near massacre, and on Apokolips Mephisto would slice through Parademons and Darkseid's hencmen/women with relative ease, then pull out a win in a drawn out battle with Darkseid. I don't believe the Omega Effect would be of much consequence against Mephisto.