Black Album first, then S&M. S&M is IMO their best cd (production wise and musically... drums and vocals do NOT sound that amazing live, especially the way Metallica mics their gear), but Black is just so damn well done, and doesn't have any *okay* songs on the album. All of them are from good to excellent. On good audio hardware the fantastic production on the Black album really shows up and can blow you away.
I agree with Eli though, all of them (outside maybe Load&Reload which are a lot alike IMO) are unique in their own way, while being pretty damn good. My least listened to Album is Kill 'Em All, but even that is still good for a play through once in a while.
