Im going to say Spartan 117, John, Master Chief, whatever you want to call him.
Running in second place would have to be Kurt, Spartan 051, because :
Exerpt from Halo : Ghosts of Onyx
The room started to spin. His mouth went dry. He tried to swallow. Couldent. His vision doubled and he thought he saw tom and lucy come back to get him... but it wasent them. It was shane, robert, and jane from team wolf pack. There were hundreds of spartans with him on the platform- from alpha and beta companies, Dante, holly, will, and even sam...all ready to fight and win this last battle with him.
Hallucination? Maybe. It was nonetheless welcome. The ghostly spartans nodded, and gave the the thumbs up "can do" signal. Kurt wouldent let them down. All he had to do was single handdedly stop a covenant army. One last impossible mission...the short definition of any spartan. It was the least he owed them.
The Fleet Master Elite snarled at kurt, and the translation filtered through his helmets speaker: "One last fight demon. You will die and we shall reopen the silver path."
"Die?" Kurt laughed. "Didn't you know?" He told the Elite.
"...Spartans never die."
Kurt turned his gauntlet face-up and pressed the detonator.
Boom went the nukes.