Hmm this is a tough one there have been alot of deaths in gundam. Especially in Zeta, SEED, and Victory. If I have to pick one. Then Mwu's death as the most memerable and recent. Although there were some more touching deaths.

Female pilots in victory were sad.
Terra's scene was a good one too.
Natarle's "sp" death. I was happy she went out that way.