That Janis Joplin version is great, I don't think I'd heard it before now. It's aspires to everything I look for in a "Cover" (in that it's almost a new composition in itself). It's original, while maintaining the original meaning and fundamental concept of the piece.
That said, I'm gonna have to say my favourite is the Ella & Louis Armstrong version:
I absolutely adore both their voices, they're so wonderfully warm and deep. It feels like a lazy summer day. While Joplin sounds great, I prefer the song in a lower register (really don't like the first link, far too harsh for the feeling of the accompaniment). Hers works because the accompaniment is changes from soft strings to a harsher guitar sound, so the voice and accompaniment are singing the same tune, to my ears anyway.
The decision is probably helped by the fact that my mum used to play Louis/Ella duets in the car when we were on summer holidays, so the combination of their voices is intrinsically linked with summertime and holidays for me
