I had to vote for the platypus after much deliberation... The wombat picture up there is somewhat unfunny which may account for its lack of votes, either way the wombat is probably more cute than it is funny.
Wombat's look like baby bears, so they don't really look too unusual. However, there is nothing on the planet that comes even close to looking like that rat-duck. Platypus all the way!
EDIT: I suppose the Kangaroo is pretty unique looking.
No respect for the bilby? It looks like the result of rabbit-on-rat action. But the best picture would have to be the platypus, I don't think it looks like it's slouching at all, it looks like he's hurling itself at some unseen target.
now here's one kick ass animal you've forgotten: the tasmanian tiger. i know it's said to be extinct but it has been seen by farmers etc, so it's not certain. and let's hope there are some left since they're pretty damn spiffy: stripes, huge maw, a pouch and a very nice tail: