Which one of these sentences is correct?
- That is one of my favourite songs of all time.
- That is one of my favourite songs of all times.
I am not a native English speaker.
I would say "of all time" because it means "from every time".
Saying "of all times" is certainly not incorrect, however, it just sounds awkward.
Check out Google's ngram viewer to see which is more popular:
And it appears "of all time" is more popular.
It's a good question about which is more grammatically correct, but the first is a common (cliched) way of phrasing it, so it is more likely to sound correct to a native English speaker, at least in the US. The second version just doesn't sound right.
"Of all time" is correct because it means "of all of time" as in the whole of time throughout existence (or throughout one's existence). Time is singular.
The only other problem in the sentence, "That is one of my favourite songs of all time," would only be a problem in the United States. In the U.S., the word is spelled "favorite."