So far I have only heard the latter one but today I've come across to the former and to my surprise the former looks like it is more common. I still feel uncomfortable using the phrase "of all time" in any sentence. It sounds incorrect but of course in fact it is the correct version.

Any ideas on this? Would you consider them interchangeable?

(There may be an American usage that I am unaware of)

However, in UK English "of all time" is the correct usage. There is no need to make "time" a plural - you could however say "of all ages", if you wished. "Of all time" literally means "since the beginning of time".

Hope this helps.

H.

"of all time" is correct - it describes a state that is indepentent of time. "This is the best music of all time."

However, you could use "at all times" to be more specific about a particular ongoing event. For example "You must be on your guard at all times, day and night."