In my humble opinion, The Jam traverse the gray area between 60s R&B and punk rock. Their 7" and 12" records end up in my U.K. punk sections, at my house. :)
When I was a young punk I didn't really listen to this band, but as an adult 35 years later, I absolutely love this band, and respect them just like I do the Undertones and Stiff Little Fingers.
Mine and probably most everyone else's favorite song: