My favourite was the Crocodile:
The fuel wasn't even in the tank, it was towed in a little trailer that you can see here at the bottom right. The flamethrower (if I remember correctly) replaced the coax machine gun, which meant that the tank could actually still fire it's cannon if it needed to.
HEEEEY-OH!!!!!
Cool.
Can't forget the ZF-1 (Fifth Element gun).