I've used all three, as well as other sample players such as Alchemy, Omnisphere, Dim Pro and YellowTools. Kontakt is easily the best of the bunch as a sample player, even though the bundled factory content is weak. The great thing about Kontakt is all the third-party content that's available, much of which is very reasonably priced.
Sampletank's aged, lacks modern features, and has a UI designed for 15" VGA displays. But I still use it on a regular basis. I predict that the upcoming Sampletank 3 will be a
huge hit. If they're wise enough to offer a can't-say-no introductory price for it, I think it'll quickly become a standard - and IKM will be forgiven for taking so long.