“Only the mediocre are always at their best.”
As quoted in The Beauty Principal (1984) by Victoria Principal, p. 117
Attributed
Help us to complete the source, original and additional information
Jean Giraudoux 14
French novelist, essayist, diplomat and playwright 1882–1944Related quotes

“My tastes are simple: I am easily satisfied with the best.”
Variant: I am easily satisfied with the very best.

“The best thing about the future is that it comes one day at a time.”

“My best birth control now is just to leave the lights on.”
As quoted in On Being Blonde (2004), by P. Munier, p. 84

“The best argument against democracy is a five-minute conversation with the average voter”
Attribution debunked in Langworth's Churchill by Himself. First known appearance is in a 1992 usenet post https://groups.google.com/forum/message/raw?msg=rec.arts.comics.misc/EMj3ZowKq1U/E0dsEBwdZEgJ.
Misattributed
Source: Google books link https://books.google.com/books?id=vbsU21fEhLAC&q=average+voter#v=snippet&q=average%20voter&f=false

“Outside of a dog, a book is man's best friend. Inside of a dog, it's too dark to read.”
This may be original with Groucho, but the Quote Investigator http://quoteinvestigator.com/category/jim-brewer/ mentions the earliest report found in a 1958 issue of Boy's Life magazine where it is attributed to Jim Brewer.
Misattributed
Variant: Outside of a dog, a book is man's best friend. Inside of a dog, it's too dark to read.
Source: The Essential Groucho: Writings For By And About Groucho Marx

This is actually James Branch Cabell from The Silver Stallion (1926)
Misattributed

“You tried your best and you failed miserably. The lesson is 'never try.'
Homer Simpson”