Platypus for sure. Were psyduck truly a duck, it'd have a little something I like to call WINGS. Really, who's heard of a wingless duck? Psyduck's clearly got four mammilian limbs, which makes it clearly a mammal.
I hate it when people are sticklers to pokemons' names. Really, why trust the word used for the creature more than its actual appearance? Psyduck's name at least always had the word, "duck" in it (it was koduck in Japan), but insisting that infernape's an ape just because of the English name? What ape has a frickin' TAIL? It's clearly a mandrill, and anyone who insists that it must be an ape because a dubber saw fit to name it after one seems pretty dense to me.
But going back to the subject of ducks, now that I think of it, the duck wasn't really done to justice in Pokemon, really. One "duck" nobody can even agree on if it's really one or not, and the one that's obviously a duck (farfetch'd) wasn't even given the water type. The duck was given the versatility of being able to both fly and swim, some ducks actually known to fly as fast as a plane, and, of course, being renowned for their swimming abilities, yet the water/flying type combination was instead given to a great, wingless sea serpent, while the only true duck was left without the aquatic inclination the duck is famed for.
But considering how pokemon treated things like the dolphin and squirrel, I think the duck family can come to forgive Pokemon.