And the winner is...

A - armpit, awkward (2)

B - baboon, badonkadonk, balls, blubbery, boobs, booger, booty, bra, buns, burp, butt (11)

C - constipated, cow pie, crotch, cup (4)

D - diaper (1)

E

F - fanny, fart (2)

G - gas, giblets, gorilla (3)

H - hot (1)

I

J

K

L

M - manure, monkey, moon, mudflaps (4)

N - nerd, nipple, nose hair, nostril, nude, nuts (6)

O

P - pee, poop (2)

Q

R - rear (1)

S - S.B.D., sexy, skid marks, snot, squat, squish (6)

T - turd (1)

U - underpants (1)

V

W - wiener (1)

X

Y

Z

So there you have it. By a landslide, nine-year-old boys have chosen B as the funniest letter.