I suppose best is Sinead O'Connor, Nothing Compares 2U.
For worst I really hate the Mad World cover by Gary Jules. I like my maudlin stuff but I thought it was very depressing. I love Tears for Fears though.
Killing Me Softly With His Song - Fugees and Lauryn Hill
The ‘In’ Crowd - Bryan Ferry
Feeling Good - Muse
Always on My Mind - Pet Shop Boys
Easy - Faith No More
Worst:
Do Ya Think I’m Sexy - Paris Hilton
A Whole New World - Peter Andre and Katie Price
Blitzkrieg Bop - The Beautiful South
Mr. Tambourine Man - William Shatner