Sourced

• A popular speaker, however unpopular and insignificant, has only to wind up his speech with half-a-dozen lines of Shakespeare (and to make it clearly understood that they are Shakespeare's) and he will sit down amid thunders of applause.
  • "Unappreciated Shakespeare", Illustrated Sporting and Dramatic News, Christmas Number, 9 December 1882

Trial by Jury (1875)

• She may very well pass for forty three
  In the dusk with the light behind her.

• But I submit, my lord, with all submission,
  To marry two at once is Burglaree!

The Sorcerer (1877)

• Oh! my name is John Wellington Wells,
  I'm a dealer in magic and spells,
  In blessings and curses
  And ever-filled purses,
  In prophecies, witches, and knells.
  
  If you want a proud foe to "make tracks"--
  If you'd melt a rich uncle in wax--
  You've but to look in
  On our resident Djinn,
  Number seventy, Simmery Axe!

Mr Wells' song, Act I

"Simmery Axe" is the traditional pronunciation of "St. Mary Axe", a road in the City of London.
In Gilbert's day, the last building was number 68, though number 70 was built later.

• Either you or I must yield up his life to Ahrimanes. I would rather it were you.

H.M.S. Pinafore (1878)

• From such a face and form as mine, the noblest sentiments sound like the black utterances of a depraved imagination!
  It's human nature! I'm resigned.
• What, never? / No, never! / What, never? / Well, hardly ever!
• Things are seldom what they seem;
  Skim milk masquerades as cream.

The Pirates of Penzance (1879)

• When your process of extermination begins, let our deaths be as swift and painless as you can conveniently make them.

• Against our wills, papa—against our wills!

• Yes, but you don't go!

Patience (1881)

• Art stopped short at the cultivated court of the Empress Josephine.

• An attachment à la Plato for a bashful young potato, or a not-too-French French bean!

• Yes, I am the Apostle of Simplicity. I am called Archibald the All-Right, for I am infallible.

• Archibald: To understand this, it is not necessary to think of anything at all.
  Saphir: Let us think of nothing at all!

• I know what love is. There was a happy time when I didn't, but bitter experience has taught me.

• A pallid and thin young man,
  A haggard and lank young man,
  A greenery-yallery, Grosvenor Gallery,
  Foot-in-the-grave young man!

Iolanthe (1882)

• The Law is the true embodiment
  Of everything that’s excellent.
  It has no kind of fault or flaw,
  And I, my Lords, embody the Law.

• The House of Peers, thoughout the war
  Did nothing in particular
  And did it very well.

Princess Ida (1884)

• I can tell a woman's age in half a minute -- and I do!

• ...a Man, however well-behav’d,
  At best is only a monkey shav’d!

The Mikado (1885)

• I am, in point of fact, a particularly haughty and exclusive person, of pre-Adamite ancestral descent. You will understand this when I tell you that I can trace my ancestry back to a protoplasmal primordial atomic globule. Consequently, my family pride is something inconceivable. I can’t help it. I was born sneering.

• …in the first place, self decapitation is an extremely difficult, not to say dangerous, thing to attempt; and, in the second, it’s suicide, and suicide is a capital offence.

• Pooh Bah: This professional conscientiousness is highly creditable to you, but it places us in a very awkward position.
  Koko: Your position is grace itself compared to that of a man in the act of chopping off his own head!

• To sit in solemn silence in a dull, dark dock,
  In a pestilential prison, with a life-long lock,
  Awaiting the sensation of a short, sharp shock,
  From a cheap and chippy chopper on a big black block!

• (Koko is negotiating the terms by which he can behead Nanki-Poo in his place, and they involve letting the latter be married to his fiance until he is executed)

Koko: But my position during the next month will be most unpleasant, most unpleasant!

Nanki-Poo: Not nearly so unpleasant as mine at the end of it.

• I have a left shoulder-blade that is a miracle of loveliness. People come miles to see it. My right elbow has a fascination that few can resist.

• Merely corroborative detail, intended to give artistic verisimilitude to an otherwise bald and unconvincing narrative.

• It's an unjust world, and virtue is triumphant only in theatrical productions.

• The idiot who praises, with enthusiastic tone,
  All centuries but this and every country but his own.

• If someday it should happen that a victim must be found
  I've got a little list, I've got a little list
  Of society offenders who might well be underground,
  Who never would be missed - who never would be missed!

The Gondoliers (1889)

• I should have preferred to ride through the streets of Venice; but owing, I presume, to an unusually wet season,
  the streets are in such a condition that equestrian exercise is impractical.

• In enterprise of martial kind,
  When there was any fighting,
  He led his regiment from behind—
  He found it less exciting.

• Of that there is no manner of doubt -
  No probable, possible shadow of doubt -
  No possible doubt whatever.

• Oh, philosophers may sing
  Of the troubles of a King,
  But of pleasures there are many and of worries there are none;
  And the culminating pleasure
  That we treasure beyond measure
  Is the gratifying feeling that our duty has been done.

• When everyone is somebody, then no one's anybody.

Utopia Limited (1893)

• Great Britain is that monarchy sublime,
  To which some add (but others do not) Ireland.

Final chorus, concluding lines

Attributed

• Humour is a drug which it's the fashion to abuse.

• I suppose he expected to see me kissing all the carpenters
  • Comment on a (possibly apocryphal) conversation between Gilbert and a French designer during the first night of Princess Ida

• No one can have a higher opinion of him than I have--and I think he is a dirty little beast.

• Take my daughters, most of whom are beauties
  • Modified libretto for the finale of Act II of The Pirates of Penzance in response to his not choosing the female chorus for a revival

• Beautiful Mabel, I'd sing if I could but I am not able!
  • Shouted from the stalls during rehearsals for The Pirates of Penzance when the actor playing Frederic was not present

• (During the period after the debut of Ruddigore, which was far less successful at the box office than its predecessor The Mikado)

Gentleman: How is Bloodygore doing?

Gilbert: The title is Ruddigore.

Gentleman: Surely that's the same thing.

Gilbert: Then it is the same to say "I admire your ruddy countenance" -- which I do -- as "I like your bloody cheek" -- which I don't.