The Giant Mouse of Minsk, in all its horrifying rotoscoped glory. Even better are the parts where the mice are shooting off fireworks at the cats. Scary, yes, but in terms of seeing the bad guys getting their asses handed to them, a very satisfying scene indeed.
Gets even better when you realize that the Giant Mouse of Minsk was essentially a tank, and the immigrants who built and piloted it were Russian.
Tiger going all badass Commando on the Cat gang near the end of Fievel Goes West. The dude was rabid!