From Paul Grundy at jwfacts:
The Watchtower claims Jehovah's Witnesses are the happiest people
on earth, without any evidence or statistics to support such an
outlandish claim. These types of comments are a type of logical fallacy referred to as a sweeping generalisation.
Watchtower 1999 10/1 p. 8 par. 13
Of course, the fact that Jehovahs servants "weep" over the
sorry state of world affairs does not preclude their being happy. On the
contrary! They are actually the happiest group of people on earth.
Watchtower 1999 10/1 pp.6-7
[Pictures on page 6, 7] Although Christians "weep" because of world
conditions ... they are actually the happiest people in the world
Watchtower 1997 May 1 p.23
[Pictures on page 23] The messengers of godly peace are the happiest people on earth today
Watchtower 1995 Jan 15 p.12 par. 7
Our close relationship with Jehovah makes us the happiest people on earth.
Watchtower 1993 May 1 p.21 par. 24
Such loving labors have made the slave class the happiest
people on earth today, and they have made millions of other people happy
too
Watchtower 1989 Mar 1 p.3
In fact, as a group, they are the most privileged, the most
successful, and by far the happiest group on earth today, as you will
see in the next article.
Watchtower 1988 Mar 1 p.17
Indeed, in every country, Jehovah's people are the best and the happiest!
Watchtower 1975 Feb 1 p.92 par. 21
The happiest people on earth are Jehovah's people and they are
chiefly among the poor as concerns the things that the world prizes
highly
Watchtower 1971 Sep 15 p.564 par. 22
All who seek a place in the redeemed society of mankind thereby
come into association with the happiest people on earth, Jehovahs Witnesses. These are the only people who know for a certainty where they
are going.
Watchtower 1962 Sep 1 p.530 par. 22
In the New World society of Jehovah's Witnesses you find people
from all walks of life, many of whom lack diplomas from mans centers of
learning; yet these people know the Bible, they are teachers. They
accept with eagerness the obligation of teaching others Gods Word. They
do it year after year with the same love of God and appreciation for
spiritual food as when they first tasted it. They are the happiest and
most peaceful persons on earth.
Watchtower 1960 Feb 1 p.74 par. 17
Now Jehovah's Witnesses are blessed above all others because they are the happiest, most peaceful and contented people on earth
Kingdom Ministry Feb 2002 p.1 par. 1 The Happiest People on Earth
"Happy is the people whose God is Jehovah!" (Ps. 144:15) Those
words describe Jehovah's Witnesses as the happiest people on earth.
Kingdom Ministry Feb 1994 p.5 par. 16
We are the happiest people on the face of the earth, knowing
that on becoming approved we will receive the crown of life. Jas. 1:12.