Cream's version of Robert Johnson's 'Crossroads'
and Willie Dixon's 'Spoonful'
Santana's version of Fleetwood Mac's 'Black Magic Woman'
Pete Seeger and other's versions of Woodie
Guthrie's 'This Land is Your Land'' as long as ALL the lyrics are sung
And one of the best covers ever is ... Jimi Hendrix's of Dylan's 'All Along the Watchtower'
IMHO it's uncommon for a cover to overtake an original,
but they do expose new audiences to not just unfamiliar songwriters, but for the curious often different genres of music also.