It's fairly subjective unless it's really lopsided. I'd argue that winning a 2/3 majority is at least a very convincing win since it's the standard for certain congressional situations. You're probably asking about Obama's re-election so 336 to 206 is the electoral college is a solid win but the difference in the popular vote there is less so. Not a fan of the obsolete and anti-democratic electoral college.
Wikipedia only lists a few landslides in US history.