Let's see....the most fearless people include: Hashirama, Naruto, Hinata, Itachi, Minato, Jiraiya, Kisame, and Kakuzu (there are obviously more)
Hashirama: Well when you summon things the size of the Juubi...you're hard to intimidate
Naruto: His will to never back down gives him the confidence to face his foes....had to fight Pain (killed his sensei), Obito with 6 Paths, and even before Madara he showed no real fear.
Hinata: How many fodders have you seen try to go solo Pain (only Hinata). Plus without a thought she was willing to take a Mokuton to the back...also doing so while being probably among the closest to the Juubi.
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Itachi: Managed to kill his family and work close to his enemy. Calm during panicking situations
Minato: Same as Itachi on situations. Died for the village and his son without a second thought.
Jiraiya: Went Pain hunting without knowing what he was getting into. Was willing to die to get his message off to Naruto.
Kisame: A dish best served cold. Ruthelessly executed allies as ordered. Never truly scared of an opponent (always that sly smile)...Sharked himself without a thought.
Kakuzu: During his entire appearance only looked intimidated once: Upon seeing the Rasenshuriken, but even regained his cool immediatley. Fought Hashirama and lived.
Though out of this crowd I'd say Hinata....simply because unlike everyone else she isn't extrodinarily strong combat wise....takes guts to go against people way above you!