All the ones I can think of atm...
I Will Always Love You originally by Dolly Parton, Whitney Houston's version is supreme.
Hurt originally by Nine Inch Nails, Johnny Cash's version is better.
Heaven originally by Brian Adams, DJ Sammy's song is better.
Black Magic Woman was originally by Fleetwood Mac (shocking I know), but Santana's version is the standard.
Whiskey in the Jar is not a Metallica song, I know Thin Lizzy covered it but they were not the original group either.