Probably my favourites would be all covers of Bob Dylan :-
Hendrix - All Along the Watchtower
Byrds - Tambourine man
Julie Driscol and the Brian Auger Trinity - This Wheels on Fire
Joan Baez - Farewell Angelina.
Oh and Dylan's cover of Paul Simon's The Boxer, and his live cover of - The Sound of Silence
Generally I prefer the original recordings.