A lot of it depends on when you're reading and which company. DC heroes usually had bunches of "name" villains, but they're not represented here so we can't show them. But here are some others:
FAWCETT: Monster Society of Evil, the Sivana Family; various individual villains such as the Black Rat, the Weeper, Dr. Macabre, the Hen, the Owl, America-Smasher, Illyria, etc.
QUALITY: Tons of costumed Blackhawk villains, most prominently Killer Shark; the Octopus; the Squid; Jasper Crow (not costumed, but the Black Condor's constant nemesis); the Minstrel; Thrilla; Jets...too many to name.
LEV GLEASON: Not only Iron Jaw and the Claw, but He-She, the Vacuum, and several others.
ACE: The aforementioned Clown plus several one-shotters. (The Clown and the Minstrel both appear to be knockoffs of the Joker.)
TIMELY: Red Skull, Python, Vulture, Byrrah, Asbestos Lady, Black Talon, Black Toad, Dr. Crime, Cat-Woman, etc., etc.
MLJ: The Skull, the Hun, a number of others.
That's just off the top of my head. If I wanted to do research, I could name a LOT more.