Smoke On The Water from Deep Purple's 1972 album "Machine Head" is probably one of the most popular songs ever for guitarists to learn. I annoyed my whole family for about a year with this song when I was 14 years old. I probably played the intro lick for two hours a day every day, it's just such a simple and classic riff! It's basically just simple double stop riffs on the 3rd and 4th string, but they're really effective because of the note placement and the space in between. It's a really great example of good song writing and how rhythm and musicality can be more important than just technical playing. The rest of the song revolves around an arpeggiated power chord shape that closely mimics what the bass is doing, which gives it a metal influence that's really fun to rock out on!