Without doubt,
Guren SEITEN gets my vote.
Motivation:
- It's
radiation arm stands out among the other KMF-weapons not only through visual appearance, but also by it's flexibility: while not only being able to directly destroy, as the other KMFs, the radiation wave can be used to trigger effects in the environment and use that to it's advantage (refer to to OotBK's battle against Cornelia at the mountain Narita in season 1)
- It's
key is not only neat but also one of the few KMF keys we get to see in detail in Code Geass
- Being really strong, the first japanese KMF and having a really vigorous pilot makes
Guren feel pretty living, despite being only a machine of destruction.
- SEITEN got an
übercool comeback scene by the middle of R2, gaining quite a number of badass points in the process (this is why I voted for SEITEN instead of the earlier version of Guren)
- Since its always of great help to the side Kallen fights for, Guren becomes
a symbol of hope whenever it's on the side your rooting for - which, imo, glorifies the image of Guren even more
- To top it off, there is
an awesome song with the same name ("Guren") appearing in R2
