There are far too many to choose just one, but the one in my sig is the one I’ll place here…
“The essential is invisible to the eye.” – Antoine de Saint-Exupéry (The Little Prince)
Never give a sword to a man who can’t dance. (Celtic Proverb, I think - it’s on my fathers chalkboard in his office)
Clothes make the man. Naked people have little or no influence on society (Mark Twain)
Early to rise, early to bed, makes a man healthy but socially dead (Animaniacs)
“Just because somebody said it, doesn’t make it true” - me
(I love quotes too, but… well… read the above again.
“STRESS is when your gut says, ‘No way!’ and your mouth says, ‘Sure, no problem!’” anonymous
Take it easy.
But take it.
–Woody Guthrie
Now THAT is a beautiful quote.
“The difference between a successful person and others is
not a lack of strength, not a lack of knowledge, but rather
in a lack of will.”
– Vincent Lombardi
“Lack of planning on your part, does not constitute an emergency on my part.”
“People often ask me my secret. I tell them it’s that I still remember how to open a dictionary.” – Cecil Adams
“Fear… those prepared to die for the truth, for as a rule they make many others die with them, often before them, at times instead of them.” - “The Name of the Rose”, Umberto Eco
Maybe it’s just me, but I’d use a ladder. 
To add some more Voltaire: “Life is a comedy to those who think, and a tragedy to those who feel.”
A few others I like:
When your passion and your contribution are the same you have found your person - Does anyone know who said this?
My sources are unreliable, but their information is fascinating - Asheigh Brilliant
Reality is for people who can’t handle drugs (Jack Handey??)
As shown by my current sig, I like GB Shaw. I’ll trow in a few more, all by him
“The power of accurate observation is commonly called cynicism by those who have not got it.”
“Democracy is a device that insures we shall be governed no better than we deserve.”
“When two people are under the influence of the most violent, most insane, most delusive, and most transient of passions, they are required to swear that they will remain in that excited, abnormal, and exhausting condition continuously until death do them part.”
“England and America are two countries separated by the same language.”
and finally
“You see things and say ‘Why?’; but I dream things that never were and I say ‘Why not?’”
“Any bloody fool can be uncomfortable” – J.B.S. Haldane
“As God said, and I think rightly,…” – either Field Marshall Viscount Montgomery -or- Margaret Thatcher.
“To love another person is to see the face of God” – the musical Les Miserables. I don’t know if it actually comes from the Victor Hugo novel or not.
If it doesn’t work, force it. If it breaks, it needed replacing anyway.
Live a poem, or die a fool.
“Those who fail to see the whimsy in things will experience rigor mortis before death.”
- Tom Robbins