Punicalagins are large molecules (ellagitannins) found in Pomegranates and Pomegranate Juice.
They themselves are very potent anti-oxidants, and they can be metabolized into other compounds (ellagic acid, urolithins) that themselves have anti-oxidant capabilities (although lesser). This large anti-oxidant value of punicalagins, about thrice that of red wine and green tea, is what brought Pomegranates to fame. That being said, the overall amount of punicalagins that get into the bloodstream after oral ingestion is quite small (6% is the highest noted and was in rats, sometimes human studies don't even notice punicalagins in the blood).
The ellagic acid and urolithins still confer a lot of health benefits, but their anti-oxidant capabilities are on par with green tea and red wine if not a bit less potent.
There is a very high variation between individuals, and like daidzein (one of the soy isoflavones) it may be genetically dependent on what intestinal microflora one possesses.