OK, I just finished watching Star Trek: Generations, and I'd have to say Kirk because on 2 occasions, he took one for the team-
1. he died in the the deflector array on the enterprise B to get them away from the nexus ribon which ended up hitting one of the sections he was in.
2. He died again when he uncloaked the rocket used by Sorron (sp?) on the bridge that broke off.
Picard gave himself to the klingons for the return of LaForge.
oh well, I still can't decide.