Replace, don't straighten. Derailleur hangers are made of aluminum, and they don't take kindly to much bending. A little tweaking with the right tool in skilled hands is okay, but why take the chance if you're working on your own bike?
It's always a good idea to keep—and travel with—a spare derailleur hanger. They're easily bent or broken, and it's something you're not likely to find in a local shop. If you're on a trip with your bike and the hanger breaks (been there, done that), your bike is out of commission.
If you can't get a hanger made by the manufacturer, the ones made by
Wheels Manufacturing are the next best thing.