1936- FDR 85% Landon 15%
I'm guessing that given the famously wrong Literary Digest poll, Landon would be doing better than that, more like 30%.
1948- Dewey 65% Truman 34 % Wallace 1%
I think Dewey does better, given how much of a shock Truman winning was, maybe 80%.
1992 Clinton 68% Bush 25% Perot 7%
I don't think any polls showed Perot close to striking distance near the end, so I think he's more like 3% and it's still a sucker bet.