I know he's out of the race, but I'll keep my first choice (and old boss) up here for a little while longer.
Joe Biden has been courageous since before he was a Senator. He marched in Wilmington with civil rights group, and he fought a tough, uphill, upset battle when he first ran for Senate as a 29-year-old. And then he had to face the death of his wife and his daughter in between winning his election and joining the Senate. He was sworn in at the hospital bedside of his two sons (who survived the accident) after an introspective search for meaning, during which he strongly considered giving up his seat. He then risked isolation from the good-ol'-boy politics of the Senate by commuting home almost every day to take care of his young children. For twenty years, he raised a family and rarely spent more than one night at a time in Washington. Many Senators do what is easy; Biden did what was right.
However, what's often forgotten is how Joe Biden stood up to Bush back in 2002. Back when everyone who disagreed with Bush was fair game, Joe objected to the Hastert Iraq War Resolution, did not vote for it in committee, and drafted the Biden-Lugar Resolution. Biden-Lugar would have forced diplomacy on the Bush administration. It could have prevented Bush's unlimited war in the Middle East. Biden had to face the fallout when the resolution was opposed by both Democrats and Republicans. Though all of our Presidential candidates who were in the Senate at the time ultimately made the wrong decision, Biden was the first candidate to stand up to Bush's Iraq plans in the Senate. That took courage.