Parliament of trees ST would take it. A well written baseline Swamp Thing pre DCnU was generally teambuster when he appears outside of single character titles. Furthermore pre DCnU he's pretty much directly contrary to Thanos being the Avatar of Death if you think about it. Honestly baseline vs baseline I think this is a pretty even fight edging out to Swampthing just because his durability is up to par with thanos and his regen/ability to reconstitute himself out of any available fauna gives him an edge. His power output is nothing to sneer at either iirc he punked superman when he was overloaded on sun amps which generally puts him into the trans tier IMO (sun-dip for extended periods that is if you consider that baseline superman is still top tier for heralds).