I'd also like to nominate the Mongoose. If memory serves me right they can be a bit mental.
Not sure if Buffalo has been nominated yet? The bit in Planet Earth II where the Buffalo holds off a pack of Lions, despite it having a huge chunk of his flesh ripped from him was pretty impressive.
Just to put it beyond any doubt, I'll nominate/second the Honey Badger (peoples champion). Would be good to see a fair competition this time around.
Wolverine - Seconded.
I'd also like to nominate the Tasmanian Devil.