Is there a general consensus on which crossword is considered the
most
difficult? How about starting a rating system? Agreed, this will be
highly subjective but it would be interesting to hear others
opinions.

Among puzzles available outside specialist puzzle
monthlies, I think you'd struggle to find anything harder
than the Listener series, formerly in the BBC's mag of
that name, now every Saturday in the Times. Tough
vocabulary, often uses non-standard clue types, and
most weeks has a gimmick to be discovered. An easy
Listener would take me at least 5 times as long
as my average for the Times.

Among national daily paper cryptics, the Guardian maybe
has the toughest hard puzzles, partly because it uses
themes and long quotes, which can make the puzzle easy
if you crack them quickly, but hard if you don't.
I'm fairly sure the puzzles in the Telegraph and FT
are noticeably easier on average than Times,
Independent and Guardian.

But any series varies in difficulty - on Saturdays,
when I do Times, Independent and Guardian puzzles,
any one of them can be the hardest. And being used
to someone's style can help a lot.

I can't see much benefit in a rating system -
experience in contests shows wide variations in
comparative times - the fact that A takes twice
as long as B to solve one puzzle doesn't prevent
him thrashing B in the next one.