Don't know a whole lot about Mongul but I seem to remember reading Hal Jordan beating him without breaking a sweat (Mongul had destroyed a city or something and Hal just took it to him)...Hal can't take out Darkseid (at least the ones I've read) and I'd put Apocalypse above Darkseid (at full potential, at least in power output) so I'm going to give this one to Apoc.
But I realize that's kind of a backwards way to get a winner, and most of it is from comics I only half remember reading (it was a while ago) so it's hardly definitive.
hhmmm..... Hal did beat Mongul...... but it cost him a broken arm..... of course this was after Mongul blew up Hal's home city and everyone he knew (almost) which seriously pissed him off........ and led to him taking out the GL corp and becoming parallax...... so he had willpower in abundance during that fight
Mongul usually puts up a decent fight against Superman
Haha...I forgot it was his home city. That would make it quite a different fight. But someone recently convinced me that Apoc could hold his own with Thanos until he has to switch bodies (and Apoc would definitely hand Supes his ass)...and Mongul isn't Thanos.