Reason number 7 of Why you may wish to think twice before you let me advise comic books unwarned:
In no particular order-
Batman villains – Clayface, Man-bat, Riddler
Superman villains – Mr. Mxyzptlk, Bizarro, Assorted Kryptonite colors
(I also love it when he or anyone wails on Nazis, the KKK or industrialists )
Spidey Villains – Doc Ock, Mysterio
I really dig Legion of Superheroes (sans Kal-el) , Avengers and other large team books, but especially when it focuses on a 2-4 member subset squad of them.
Hulk stuff – absorbing man, books where Hulk and Bruce work together , Marie Severin art
Captain America / Nick fury crossovers
Grant Morrison Animal Man / Doom Patrol… but not his recent stuff so much.
Silver age type stuff for the most part. If it is current stuff in 60s style with great subtext, even better.