FSU is probably the easy winner. They were ranked #10 in week 1 and are now 2-9
Honorable mentions go to:

Utah (4-7) ranked #10 in week 5
Michigan (6-5) Ranked #9 in week 1
Oklahoma State (3-8, soon to be 3-9) ranked #13 in week 2
USC (6-5) ranked #13 in week 2
Arizona (4-7) ranked #21 in preseason poll
Oklahoma (6-5) ranked #16 at one point
and of course 5-6 Kansas who was ranked #19 early in the season.

Mizzou needs to go on the list. They're not having a terrible season at 8-3, but they were not anywhere close to the #6 team in the country when they were ranked there earlier in the season.