In the mid to late '80's, I read nothing but G.I.Joe. By far, that was - and is -my favorite comic book of all time. But then in the early '90's, I got into all the superhero comics and I had a
lot of interests.
For the big two (Marvel and DC), I would say Silver Surfer, Gambit, and Adam Warlock ranked among my favorites for Marvel, while Aquaman and Robin were it for DC.
Villian wise, nothing beat Thanos for me, especially in the Infinity Gauntlet series.
Outside of the two powerhouses, I would say Spawn was probably one of my favorite heroes. Sadly, I stopped reading comics after awhile and never really saw how Spawn played out, but, initially, at least, it was a fantastic comic and easily my favorite.