Hmm.. I would normally say Wombat, but I just saw a sequence on that Planet Earth show about a bunch of lions taking down an elephant. A wombat just seems too boring in comparison.
Looking at the tale of the tape, the Wombat is just outmatched by the lion. Speed, reach, claws, strength, agility, it's not a close match. The wombat would be tapping out, or worse, to the lion within 5 minutes.
Gotta go with the Lion. Although, if you took the genetic material of both wombat and lion, and created a limbat or a wion, that would probably trump both.