basically any of supes rogue's gallery. other than savage dragon superman is the only guy i know who most of his villains are strong enuff to go toe to toe.
i understand cosmics to be the abstarcts, but i'll still leave out darksied, and any from apok since he's a god:
Grundy
bizzarro
gog
ultra humanite
metallo
brainac
eradicator
Grayven(son of darksied but demi-god)
Draaga
mongul
mongul II
mongal
Maxima(after he had just turned her down, he became sworn enemy)
lobo
a rogue daxamite or two
doomsday
the cyborg
preus
sometimes Grodd(whose thrice in size and strentgh now)
gen. eiling(though he's more a JLA guy)
replikon
white martians
parasite
sometimes black adam
when it comes to sheer brute stentgh these are the folks you look to.