I'm actually surprised not a single person replied to this >_> Guess no one likes Godzilla.
Anyway, Godzilla always wins against Mech-Godzilla. But it really does depend on which Godzilla and which Mech-Godzilla is fighting. The newest Mecha-G from Tokyo SOS would most likely be able to beat 1954 Godzilla all the way up to the 90's Godzilla. But not the new Millennium Godzilla.