“If you are allergic to a thing, it is best not to put that thing in your mouth, particularly if the thing is cats. -Lemony Snicket”

The Wide Window (2000)

Conversation with Queen Victoria after a Royal Command performance of The Gondoliers in March 1891, the 'gags' in question are ad libs added by the actors during the performance
Quoted in The Complete Annotated Gilbert and Sullivan, Ian Bradley, OUP, 1996. Originally found in the magazine The Era

Source: Valiant

Variant: You your best thing, Sethe. You are.
Source: Beloved

Douglas Adams. The Salmon of Doubt: Hitchhiking the Galaxy One Last Time. New York: Random House, 2002, 135–136.
Also quoted by Richard Dawkins in his Eulogy for Douglas Adams (17 September 2001) http://www.edge.org/documents/adams_index.html
Context: If you try and take a cat apart to see how it works, the first thing you have on your hands is a nonworking cat. Life is a level of complexity that almost lies outside our vision; it is so far beyond anything we have any means of understanding that we just think of it as a different class of object, a different class of matter; 'life', something that had a mysterious essence about it, was God given, and that's the only explanation we had. The bombshell comes in 1859 when Darwin publishes On the Origin of Species. It takes a long time before we really get to grips with this and begin to understand it, because not only does it seem incredible and thoroughly demeaning to us, but it's yet another shock to our system to discover that not only are we not the centre of the Universe and we're not made by anything, but we started out as some kind of slime and got to where we are via being a monkey. It just doesn't read well.

Radio From Hell (August 21, 2007)
Variant: I want to put everything in my mouth.

Source: The Odyssey

Source: The Road

As quoted by John M. Kost http://www.mackinac.org/bio.aspx?ID=104 (25 July 1995) in S. 946, the Information Technology Management Reform Act of 1995: hearing before the Subcommittee on Oversight of Government Management and the District of Columbia of the Committee on Governmental Affairs (1996).
This appears to derive from a 1910 advertisement by writer Alfred Henry Lewis for a forthcoming series of biographical articles about Roosevelt: "All activity, Mr. Roosevelt has often shown that it is better to do the wrong thing than do nothing at all. In politics this last is peculiarly true. The best thing is to do the right thing; the next best is to do the wrong thing; the worst thing of all things is to stand perfectly still". (e.g. in La Follette's Magazine https://books.google.com/books?id=RV4CAAAAIAAJ&pg=PA183&dq=%22best+thing%22+%22right+thing%22+%22worst+thing%22+nothing&hl=en&sa=X&ved=0ahUKEwjNksu-nZrMAhVDy2MKHSl1Df8Q6AEIIzAB#v=onepage&q=%22the%20best%20thing%20is%20to%20do%20the%20right%20thing%22&f=false (28 May 1910)
Disputed