"But Cilla's version outsold Dionne's in the UK."
Yes, but in this case I don't think Dionne's was officially released domestically in the UK (was it?)

Regardless, they were saying "best" :P
The Zombies have a great cover of Will You Still Love Me Tomorrow--I agree with you about King's and the original Shirelles. (Actually The Zombies also have a great cover of The Look of Love, though nothing tops Dusty's for me.)
I know this is predictable from me, but as much as I love Jimmy Webb's production for Richard Harris, I would much rather listen to Donna Summer's MacArthur Park. And I would MUCH rather listen to her disco-fied Could it Be Magic than Manilow's.
Howabout Bette Davis Eyes? I prefer Carnes synth version to the country-ish original by Jackie DeShannon (who wrote it.)