I'll admit it, I've played through every Quake and official expansion pack available. I own Quake III; Yes I'm a bad person but now I can make amends by purchasing the whole series (minus IV but no one liked it anyway) for a mere $14.99 this week only. I will probably buy it for a few friends as well, just to make it up for my previous transgressions.
You will want to grab the Ultimate Quake Patch and the Ultimate Quake Patch II as well due to the updating that these free patches provide. You could also roll your own by grabbing a source port and the music addons from the FAQ page.