The reishi mushroom (Ganoderma lucidum), which is also known as wild lingzhi, has become one of the most talked about in alternative medicine in recent years, largely due to the amount of anti-cancer research involving it.
Most fungi have very little antioxidant activity. For example, your typical white mushrooms have an ORAC value of 691. Shiitake and crimini have more, but even the most potent commonly consumed dietary source, portobello, is still only 968. The fact that the reishi mushroom has 10 times more antioxidants by weight is impressive. For every 1 gram of dry weight, the free radical scavenging activity has been reported as:
9.2 mg Catechol Equivalent for total phenolic content
2.14 mg Quercetin Equivalent for total flavonoid content
That being said, it's important to consider that the reishi was measured dried and any dried plant (or fungus) will have an exponentially higher value since the water has been removed and the active ingredients are more concentrated.
While its high ORAC is a nice bonus, the health benefits of mushrooms have little to do with their antioxidant content. While this species comes in many different colors including the black reishi (Ganoderma sinensis), almost all of the scientific research suggests it is the red reishi which offers the most medicinal benefits. The research about reishi mushrooms and cancer will explain further.
Research Support, Non-U.S. Gov't: The Journal of Alternative and Complementary Medicine. Volume 13, Issue 1. NIH 2007