The guy held back Superman before and has beaten Etrigan. Combined with the fact he can encase Gladiator in plant matter. I would say Swamp Thing takes a majority.
Are you talking about that time Swamp thing calmed an enraged Superman down?
That wasn't a fight. It was mostly just Superman knocking Swamp Thing around for a few pages and then helping Clark to go into a deep sleep until he recovered from whatever illness was making him destroy everything.