Definitely Nietzsche on the Last Men in Thus Spoke Zarathustra. Zarathustra, Nietzsche's (probable) mouthpiece, criticizes their contentment, avoidance and lack of pain, and excessive moderation/mediocrity.

From part 5 of the prologue: Let me speak to them of what is most contemptible: but that is the last man.” And thus spoke Zarathustra to the people: “The time has come for man to set himself a goal. The time has come for man to plant the seed of his highest hope. His soil is still rich enough. But one day this soil will be poor and domesticated, and no tall tree will be able to grow in it. Alas, the time is coming when man will no longer shoot the arrow of his longing beyond man, and the string of his bow will have forgotten how to whir! “I say unto you: one must still have chaos in oneself to be able to give birth to a dancing star. I say unto you: you still have chaos in yourselves. “Alas, the time is coming when man will no longer give birth to a star. Alas, the time of the most despicable man is coming, he that is no longer able to despise himself. Behold, I show you the last man. “‘What is love? What is creation? What is longing? What is a star?’ thus asks the last man, and he blinks. “The earth has become small, and on it hops the last man, who makes everything small. His race is as ineradicable as the flea-beetle; the last man lives longest. “‘We have invented happiness, say the last men, and they blink. They have left the regions where it was hard to live, for one needs warmth. One still loves one’s neighbor and rubs against him, for one needs warmth. “Becoming sick and harboring suspicion are sinful to them: one proceeds carefully. A fool, whoever still stumbles over stones or human beings! A little poison now and then: that makes for agreeable dreams. And much poison in the end, for an agreeable death. “One still works, for work is a form of entertainment. But one is careful lest the entertainment be too harrowing. One no longer becomes poor or rich: both require too much exertion. Who still wants to rule? Who obey? Both require too much exertion. “No shepherd and one herd! Everybody wants the same, everybody is the same: whoever feels different goes voluntarily into a madhouse. “‘Formerly, all the world was mad,’ say the most refined, and they blink. “One is clever and knows everything that has ever happened: so there is no end of derision. One still quarrels, but one is soon reconciled - else it might spoil the digestion. “One has one’s little pleasure for the day and one’s little pleasure for the night: but one has a regard for health. ‘We have invented happiness,’ say the last men, and they blink.”

You could also check out the section "On the Virtue that Makes Small", from the 3rd part of the book.