In terms of badassery, Kisame. The man commited suicide trapped in a water sphere eaten by sharks. Damn.
In terms of major plot progression, Minato and Kushina. They died sealing Kurama into Naruto, saving Konoha, and stalling Madara's I.T plan drastically.
In terms of sacrifice, Itachi. This one is pretty self-explanatory.
Sorry Jaraiya, but your death was... meh.