If you look back at the last 110 years of Presidential elections, many point to Ronald Reagan's overwhelming defeat of Walter Mondale in 1984 as the biggest landslide of all time. Indeed, Mondale did take a shellacking, winning only one state (his own) with the District of Columbia (basically a Democratic consolation prize) receiving a mere 13 electoral votes to Reagan's 525.
With only 2.4% of the electoral vote, Mondale's electoral count sets the pretty bar low. However, he can rest assured that he was not the biggest loser in a modern Presidential election, electorally speaking. That unique title goes to the long forgotten Republican candidate of 1936, Alf Landon.
Alf Landon was the governor of Kansas when he was nominated to run a fools errand campaign against the ever-popular Franklin Roosevelt in 1936. The reason for his nomination? He was the only Republican governor to have been re-elected in the midterms of 1934, which an overwhelming Democratic year. Unfortunately for the Kansas Governor, the underwhelming campaign was doomed from the start, even the leadership of his own party even referred to him as "the poor man's Coolidge".
In the end, Landon did win 2 states, Maine and Vermont, which was one state more than Mondale. But the electoral vote count only added to 8 (to FDR's 523), which is 4 less than Mondale, a mere 1.5% of the electoral vote total compared to Mondale’s 2.4%. Each calculated against its respective total electoral vote count.