Gurren Lagann vs GaoGaiGar- this would be the greatest fight ever! I could imagine Hell and Heaven vs Giga Drill Breaker.
and other Jesus Yamato (Strike Freedom) vs Amuro Ray (Hi-Nu Gundam)- I just want to see Amuro beating up Jesus Yamato
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Gunbuster vs Buster machine 19- why 19? Because Diebuster would be unfair for Gunbuster.
Cho Gigna Gurren Lagann vs. Diebuster- now to see two of the largest robot in anime fighting each other is something to watch.