What is the best way to remove a rounded bolt?

If the head is completely rounded beyond hope, you will have your best chance of success by using a bolt extractor. A common brand name is "Easy Out".

There is a pretty good how-to at the link below. One thing I recommend is when you drill the hole, use a moderately dull bit to enlarge the pilot hole. That will cause the bolt to heat up and take in the penetrating oil that you have squirted on the threads.