These are all the ones that I'm sure of:
3) Megator (1st death; later on he is resurrected as a zombie)
4) King Grayskull
10) zombie Megator (2nd death)
13) All Snake Men (including Kobra Khan, probably Snake Mekaneck, and presumably all the peaceful Reptons as well, since they are the same species as Khan)
There are definitely more, as many heroes are said to have fallen during the second ultimate battleground war, but they have yet to be revealed.