http://www.english-test.net/forum/ftopic22594.html
[T]he article can sometimes be omitted. It is often dropped in casual English
when it is not the subject of the clause, or when the noun phrase is not
otherwise modified:
The happiest man is the single man.
The single man is happiest.
The single man is the happiest man I know.
Also, it is dropped, as in your example, when the comparison is of the same
entity at e.g. different times:
The weekend is when I'm busiest.
It's hottest in the summer.
And then, of course, there is no necessary 'the' when most means very:
A single man is most accommodating.