Criteria:  The candidate must be dead (for some years, to be polite);
there must be written evidence to back up the claim (anything from
private correspondence to a public accusation through the media); the
proof must be colorfully and convincingly expressed (with hard
evidence given for the charge); yet the putative bore need not be
famous (he or she can be someone's otherwise anonymous relative just
as plausibly as a president, prime minister or prelate).

How odd that a question about dullness should be so interesting. ;-)

I have lots of ideas, from Calvin Coolidge to Anna Nicole Smith, but I
can't back up my notions with hard data. (Yes, I know that Anna Nicole
is not dead, unless we are talking about intellectual capacity.)

I would like to pass along this quip that I found:

"Jodie Foster told an interviewer that her ambition in life is to be
the most boring person on earth. So now Al Gore could be runner-up in
two races."

The Zany Reports
http://www.bobzany.com/zanyreports/01042000.htm

According to the Public Record Office of Northern Ireland,
Lord Charleville of Tullamore left an heir "whom Creevey
had described in 1833 as 'justly entitled to the prize as
by far the greatest bore the world can produce'".

It does not say anything about the identity of 'Creevey',
or Creevey's justification for the remark.
http://www.proni.gov.uk/records/private/howbury.htm

THE BORING INSTITUTE is dedicated to this very subject.

http://www.boringinstitute.com/

The World of fiction gives us Marvin the Paranoid Android! You might
try listening to some Vogon poetry while you're at it.