Probably no one will agree but,
Voivod cover of Pink Floyd's "Astronomy Domine".
https://www.youtube.com/watch?v=24Q01-TWa-w
I agree. The Voivod cover is excellent (but still very close to the original)
My entry:
Donna Summer - State of Independence
(which is a cover of a Jon and Vangelis song) that sounds like a demo compared to Donna's version.
Trivia: "Donna Summer covered Jon Anderson & Vangelis State of Independence on her 1982 album Donna Summer produced by Quincy Jones. Summers version of the song features an all-star choir including among others Michael Jackson, Brenda Russell, James Ingram, Dionne Warwick, Kenny Loggins, Lionel Richie and Stevie Wonder."