Fatypus used to boast that the Alotta was the fattest ski on the market...but i think it's only 140 at the waist, making it only the 3rd fattest now (i think once you make a ski and boast that it's the fattest that opens the floodgates for somebody else to come along and make a fatter one).
Also considered pretty fat:
Praxis - (136 waist)
DP 138 (138 waist)
Moment Comi-Kazi (136 waist)
Though the 136-138 waist seems to be replacing the old 120-125 waist as the norm for super powder skis.