The biggest landslide victories in Presidential Election history since 1900 would be the following in chronological order:
The Election Of 1904–Theodore Roosevelt vs Alton B. Parker
The Election of 1920–Warren G. Harding vs James Cox
The Election of 1924–Calvin Coolidge vs John W. Davis and Robert La Follette Sr.
The Election Of 1928–Herbert Hoover vs. Alfred E. Smith
The Election of 1932–Franklin D. Roosevelt vs Herbert Hoover
The Election of 1936–Franklin D. Roosevelt vs Alf Landon
The Election of 1964–Lyndon B. Johnson vs Barry Goldwater
The Election of 1972–Richard Nixon vs George McGovern
The Election of 1984–Ronald Reagan vs Walter Mondale