It's funny when some people dismiss them as a pop band. They were very diverse in their musical tastes which was reflected in their songs. They were always trying to come up with something new and to not repeat themselves. (As opposed to today's music which seems to depend on algorithms that guarantee repeating yourself.) Not only were their hairstyles changing year to year, but also the music.
The Beatles had plenty of rockers and heavy sounding songs - like Helter Skelter and Tomorrow Never Knows. But i think their first heavy sounding song was Ticket to Ride. Listen to Ringo on the drums and Paul and George dueling lead guitars.