When one looks back in American history at losing Presidential candidates who should have been President in their times, three names stand out:
Henry Clay of Kentucky, 1824, 1832, 1844
Charles Evans Hughes of New York, 1916
Hubert Humphrey of Minnesota, 1968
All three of these Presidential candidates were exceptional public servants.
Henry Clay was the most important legislator of the first half of the 19th century, known as the Great Compromiser, for his promotion of the Missouri Compromise of 1820, the Compromise Tariff of 1833, and the Compromise of 1850. He was the youngest Speaker of the House of Representatives, Secretary of State, and also served a number of terms in the US Senate. He stood for a stronger national government, in the Alexander Hamilton model, and had a great impact on many others including Abraham Lincoln, who became a Whig Party member due to the influence of Clay on him.
Charles Evans Hughes was the progressive Republican Governor of New York, in the Robert LaFollette-Wisconsin model in the early 20th century, served as an Associate Justice of the Supreme Court, resigned to run for President against Woodrow Wilson in the closest electoral vote election since 1876, and third closest electoral vote election of all time, and then went on to be Secretary of State. Finally, he became Chief Justice of the Supreme Court in the 1930s, the New Deal era.
Hubert Humphrey was the Democratic Senator from Minnesota, a leading liberal figure, who had been Mayor of Minneapolis before going to the Senate. He was seen as a premier liberal in Congress, responsible for many of the ideas that became the Great Society. He was Vice President under Lyndon B. Johnson, unhappy in that position and forced to support the Vietnam War in speeches, which undermined his Presidential campaign in 1968 against Richard Nixon. He came back to the Senate after his Presidential defeat, and sadly died at the young age of 66 in 1978.
If these three losing Presidential candidates had won, the history of the United States would have been vastly different.