Which spark plug should I get? Is there only certain ones that fit?

So I replaced my plugs on my Volkswagen a few years ago, but since I moved up here to the northwest from the coastal southeast, my fuel economy has dropped and it's starting to annoy me so I'm thinkin bout checking and/or replacing the plugs for maybe better ones. Question is, I know different cars require different heat ranges and gap, and I don't remember gapping the current ones at all...maybe they already were in the box I don't remember, but what would you say is the best plug for the best burn and economy possible?