The late Jeff Buckley's haunting version of Leonard Cohen's magnificent "Hallelujah" (1994), it's one of the rare exceptions where the cover version is vastly superior to the original. Which is saying a lot, as I love Leonard's original version as well, but Jeff transformed this song.
The studio version is by far my favorite, bittersweet and brooding. Just his angelic voice and a melancholic electric guitar. Listen here:
And one more, live in studio. Great to watch him perform, but it's not quite up to the same level as the album version posted above.
"Hallelujah" has been covered many times by many musicians since and will be covered again, but I don't see how anyone could top Jeff's perfect version. And if you don't have his album Grace, buy it now.