Reading through an old issue of Black Panther, I came across this panel:
(from Black Panther (1998) #56).
This is a pretty obvious reference to Batman villains Penguin and Riddler; it's not even a bit subtle.
Is this just a one-off gag written into this issue, or have other Marvel characters made references to fictional (within the universe) characters that are obviously meant to be Batman, or other DC heroes/villains?