In Skyrim they're voiced by King Bumi so they're automatically better.
Khajjit: Skyrim wins
Argonian: Oblivion wins
But overall I think Morrowind did both of them better (and that was with minimal voice acting.)
They sound far better in Skyrim. That's my opinion....for what its worth.
As far as Khajiit voices go I like Morrowind, then Skyrim, then Oblivion.
Skyrim easily, for me at least. They're so damn soothing to listen to, especially the males. Considering their reputation as thieves, I actually feel welcomed just hearing "Khajiit has wares, if you have coin."
Oh I dunno, they can sound pretty aggressive sometimes. Recalling the skooma addict that killed his brother in the Alftand glacial ruins when he first sees you.
Gotta say though, Oblivion Khajiit/Argonians sound cuter.
They were voiced by the same person in both Morrowind and Oblivion, with him doing the same voice for Argonian males.
It is possible to prefer one interpretation over the other. Think of all the actors who have played Hamlet more than once. Was each interpretation equally successful? Of course not.
Just as, say, someone might prefer Peter O'Toole's Henry II in Becket over his interpretation of the same character in A Man For All Seasons, so we might prefer an actor's work in Morrowind over his work in Oblivion.
It's all good. That said, I find the Skyrim Khajiit voices and the Oblivion Argonian voices to be wonderfully soothing and a pleasure to listen to.