In an interview with Rolling Stone, Kirk Hammett talked about hanging out with Kurt Cobain. 

Is it true that the album cut "Whiplash" was Kurt Cobain's favorite Metallica song?

Absolutely. He told me that himself. He came to one of our shows in Seattle, on the Black Album tour. I remember at one point, we were playing "Whiplash," and he looked at me and kept punching the air with his fist, and gave me a big thumbs-up sign. I was like, "Cool. Kurt, I know you love this song. This one's for you!" I knew Kurt kind of well, and I hung out with him quite a bit. He was a pretty big Metallica fan – I was surprised at how much of a Metallica fan he was. He loved Ride the Lightning. He loved that album.

Kirk Hammett talks about his new book and new Metallica HERE.