Notable opponents include:
Silver Banshee, Cyborg Superman (Zor-
El), The Bizzaro version of herself, Reign, Psycho-Pirate, Brainiac, Clayface, Dollmaker, Gorilla Grodd, Harley Quinn, a selection of Darkseid's minions, Lava Girl, Killer Croc, Live Wire, Metallo, Mongul, Solomon Grundy, and the MAIN MAN, Lobo.
Other possible contenders include Giganta, Killer Frost, Bizzaro, Lex Luthor, Drax (not to be confused with Marvel's Drax), Jax-Ur, Nadira, Zora Vi-Lar. All are either earrth villians who would put up a good fight, or are known kryptonian prisoners in the phantom zone.