Marlon Asher - Ganja Farmer, Afroman - colt 45 (2 Zig zags)
Anything from the 'throw me herb riddim' album by Gappy Ranks is good, it's all about farming weed and smoking it so it's pretty appropriate really
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Almost forgot to mention, any instrumentals from Dr.Dre's 2001 album are good. Sometimes you don't need lyrics, just a nice west coast beat "AFN smoke out"