The sound of Fretless Bass became one of my most favorite sounds. This instrument usually creates that smooth “bwoww” sound but it can also create a sad, crying "muwahh" sound that is good for some sad parts in the songs. Fretless bass is most often used in death metal - progressive and technical death metal, in particular. Fretless bass is harder to play than the fretted bass so the guys on this list are very talented. And I guess they practiced a lot. Talent + hard work.