4. Swamp Thing Vs Moss Man
A battle between these two green beasts would be epic! Both of them have the ability to control plant-life, so watching them both utilising this ability during their confrontation would be fantastic - tangling each other in vines, using the branches of massive trees as giant hands, bringing to life the very ground beneath each other and using it as a weapon. With each character fighting on the side of good in their respective universe, there would have to be a creative reason to have them fight against each other, but that's hardly a problem - it's not as if heroes have never fought each other in the past! I'm sure they would eventually settle their differences amicably, but an initial greenery-based duel would be a sight to behold.