If the RX-78 Gundam is the signature mobile suit of the Gundam world, then the Zaku II is a close second. The Zaku II is the main weapon of the Zeon empire. In fact, the Zaku II is the first mobile suit seen at the begining of the animation and has shown up in just about all the other Gundam series, (as variants.) You can't really call it the Gundam's rival though as Zakus get destroyed quite regularly.