I was going to say "villain teamup" stories, but some stories may bring in two villains and have them fight one another (and maybe the hero) for the whole story.
Some of the best include:
Debut story of THE SINISTER SIX
The first JLA/JSA crossover, in which the "Crime Champions" of two worlds collaborate against the heroes
First Luthor-Joker teamup in WORLD'S FINEST
The multi--part MONSTER SOCIETY OF EVIL series in CAPTAIN MARVEL
The first (and only?) Joker-Penguin teamup in comics
Either the first or second Luthor-Brainiac teamup
Kingpin crosses swords with the Schemer
The Owl battles Doctor Octopus
An appearance of his Rogues Gallery was always a highlight of reading The Flash in the Silver Age. It was entertaining to witness the clash of egos on display between the rogues. This was particularly so when you considered that Barry should have been able to defeat them all before they even knew he was there.
Here's one of the earliest.
Technically, by the time Kree-Nal and Cat Girl did that JAGUAR cover together, they'd both got out of the villain game in the hopes of impressing the studly hero. But hey, former supervillains is a legit subcategory.