“Butter” can mean a food or “someone who butts.” A riddle is:
 
Q: What is the best butter in the world?
A: A goat.
 
Forms of the “butter” pun have been cited in print since at least 1901.
 
   
16 June 1901, Los Angeles (CA) Sunday Times, pt. 2, pg. 1, col. 5 ad:
What is our best butter? Well, we’ve got all kinds except—a goat.
(...)
GURNEY GROCER CO.
 
3 September 1909, Washington (DC) Post, “Pointed Paragraphs” (from the Chicago News), pg. 6, col. 7:
The best butter isn’t strong—unless it’s a goat.
 
22 February 1925, Dallas (TX) Morning News, “The Riddle Man Offers Few More Brain Teasers,” pt. 7, pg. 7, col. 1:
What is the best butter in the world?
A goat.
 
29 December 1928, Chicago (IL) Defender, “Teasers,” pt. 2, pg. 3, col. 2:
What is the best butter?
A goat.
 
Google Books
A Treasury of Laughs for Boys & Girls
By Robert C. Mellon
New York, NY: Hart Publishing Company
1957
Pg. 86:
What is the best butter in the world?
A goat.
   
Google Books
Taking Sides
By Norma Klein
New York, NY: Avon Books
1976, ©1974
Pg. 56:
“What is the best butter in the world?” Seymour asked.
 
“A goat!” Hugo yelled and raced into the house.
     
18 December 1996, The Sun (London, UK), pg. 6:
What’s the best butter in the world? A goat!
 
Twitter
Yorry Fremulon‏
@YorryFremulon
What do you call the best butter on the farm? A goat. #FunIdiom

9:37 AM - 14 Jan 2017