Growing up my favorite thing about comics was not who was writing it or who was drawing it...it was what two superheroes where fighting each other.
This Action Comics annual is one my all time favorites. I would read it and re-read it over and over again. The idea of Captain Marvel having to fight an Eclipsed Superman is just an amazingly fun idea.
It's been done to death since and not even the story in DC Comics Kingdom Come comes close to this issue.
Other good fights are,
(issue numbers unknown...but I read them, I swear)
Hulk and Thor
Wolverine and The Punisher
Spider-Man and Daredevil
Ultimate Thor and Supreme Power's Hyperion
Invincible and Allen The Alien