The original Chaka version was from 1974, with this remix from 1978.
The Whitney version has a superb remix by C&C and came out in 1993.
Which is best?
IMHO they are both great, but I think the Whitney version just edges it, mainly courtesy of the great C&C remix and as good as Chaka's vocals are, nobody can out-sing Whitney!
Chaka Khan- I'm Every Woman
(12" Club Mix)
Whitney Houston - I'm Every Woman
(Clivilles And Cole House Mix 1)