As one of the most powerful heroes in the DC Universe (and all comics), Superman has taken on evil gods, reality warpers, and some of the smartest and most dangerous people on the planet, but surprisingly Poison Ivy is the one villain who is shown to consistently be able to beat the Man of Steel with no issue.
Superman is routinely one of the first lines of defense against any threat to Earth. The villains Superman regularly fights are evil geniuses like Lex Luthor, monsters like Doomsday that grow stronger from death, energy parasites, and alien warlords such as Mongul, yet Superman regularly manages to defeat all of them. Superman's powers are truly godlike, and yet Pamela Isely has managed time and time again to easily defeat Superman, not through sheer brute strength but by the nature of her powers. In fact, Superman has lost to her more regularly than any of his regular villains.
Readers get to see Poison Ivy's secret weapon against the Man of Steel in Trinity #2 by Francis Manapul. It's well known that Poison Ivy has control over all plant life, and while people might assume that control is limited to plant life on Earth, that isn't the case. Ivy shows absolutely no issue controlling the Black Mercy, an alien plant that can trap people in hallucinations. The Black Mercy is the Justice League's biggest threat, slipping its victim into a dream of their perfect life while feeding off of their energy. This dangerous plant already defeated Superman in the famous storyline For the Man Who Has Everything by Alan Moore and Dave Gibbons. Not only was it able to enthrall him, but when he was pulled from his dream of a perfect life, he went into a rage and nearly killed Mongul in a fight. The fact that Ivy seems able to control the mind-altering plant at will means she already has one of Superman's greatest weaknesses at her beck and call, but this isn't the first time she's defeated the Man of Steel without issue.
Poison Ivy Easily Defeated Superman, Batman, And Wonder Woman
Poison Ivy has defeated the Man of Steel with ease several times. One of the most well-known examples was in the classic storyline Batman: Hush by Jeph Loeb and Jim Lee, where Poison Ivy was able to use Kryptonite to take control of Superman's mind. In a story arc during writer Tom King's run, Poison Ivy gained control of the Green, which allowed her to control just about all of humanity, with Superman included. Poison Ivy's powers are terrifying but often overlooked by most comic fans. While her control over plant life certainly makes her strong, few realize just how many times she's gone up against Superman and won with complete ease.
The villains Superman fights regularly rank among some of the most dangerous villains in the entire DC Universe, but every time Superman comes up against Poison Ivy, the Man of Steel can never seem to win.