Fire on the Mountain by the Grateful Dead runs on a B to A chord movement, and that one chord change opens up B Mixolydian, which is one of the most fun scales to explore if you've never spent time with it. Think B major pentatonic as your starting point and just add a couple of extra notes.
What makes this riff so interesting is that the coolest line in it sits slightly outside the scale, and it works anyway. That's kind of the genius of how the Dead approached things. It pulls from jazz vocabulary, a descending chromatic-flavored run that has no business sounding that good over a rock groove, but it absolutely does.
If you've been living in minor pentatonic and want to hear something completely different, this is a great place to start. Go have fun with that one.
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