Pancho & Lefty is a great story song. But even though the Willie and Merle version is the most famous it's really not the best rendition of it. I think if you want a great singer's version, listen to Emmylou. However, I think listening to Townes Van Zandt sing his own song, despite not being remotely a great singer, is the best.
It's like listening to Kris Kristofferson. Kris is not remotely a great singer. And, generally, I'd rather hear someone else sing his songs than him... but there's some that work for Kris' voice. It's the same with Townes. He's not a great singer but there are