Hmm, a tough one. Roosevelt '36 had the biggest win in the electoral college (523-8; 98.49%) and the biggest vote margin (24.25%); Johnson '64 had the largest share of the overall vote (61.05%); Nixon '72 won the biggest vote margin (17,995,488); and Reagan '84 won the most votes (54,455,472) and the most electoral votes (525).
There is no clear answer to this, without specifying exactly what kind of statistic determines a 'landslide'.
I call a landslide overall victories for one political party. That includes PV, EV, as well as congressional and state elections; that was why I voted for FDR in '36, because of the overall victory.