"I caaaaan't staaand him." "I cain't stannim!" "No, no, no, caaaaaaaaan't." "Caiiiiiiiin't."
"Well, I can't make love to a BUSH!"
The following, immediately after filming a Big Damn Kiss scene for The Dueling Cavalier:
Lina: "Oh Donny! You couldn't kiss me like that and not mean it just a teensy bit!" Don: Meet the greatest actor in the world! I'd rather kiss a tarantula. Lina: You don't mean that. Don: I don't— Hey Joe, bring me a tarantula!
The entire "Make 'em Laugh" song number. Every. Single. Thing! Including Cosmo's intro.
Cosmo: In the words of that immortal bard Samuel J Snodgrass, as he was about to be led to the guillotine! ...MAKE UM LAUGH!
"Moses Supposes"—again, including the build-up, from the moment Cosmo trots in: his saving Don from having to repeat a ridiculously long passage...by congratulating the coach and "encouraging" him to say another one (guess which one!)—and then troll-facing the man all the way (the punch line being when the coach catches him)!
The whole scene with the "All I Do is Dream of You" number from Kathy popping out of the cake and realizing that Don was right there (having previously brushed off Don) to Don snarking about her being a serious actress. At the end, when Kathy is trying to brush off Don again:
Don: Now don't go... now that I know where you live (gestures to the cake), I want to see you home.
In the "Broadway Melody" sequence, we see Gene's character rising from cheap burlesque houses to the most upscale venues - with him and the chorus girls doing the exact same number.
When The Dancing Cavalier is a smash, and Lina starts reminding R.F. of the promises he made to her, which irritates Don, Cosmo and Kathy to no end. When R.F. seems to be caving:
Cosmo: Simpson, I once lent you a cigar. Can I have it back?
R.F. announces all the films at the studio henceforth will be sound pictures.
Cosmo: That means I'm out of a job. At last I can start suffering and write that symphony.
R.F.: You're not out of job, Cosmo, we're putting you in as head of our new music department.
Cosmo: Why, thank you, R.F. At last I can stop suffering and write that symphony.
(about Lina) "She can't act, she can't sing and she can't dance. A triple threat!"
If you weren't aware, in the entertainment industry a "triple threat" is someone who actually can dance, sing, and act. More information at The Other Wikihere. Cosmo is inverting the definition for humorous effect.
Basically everything Cosmo Brown says, but particularly his 'Jazz Singer' impression.
When the company first decides to make a Voiced Production. They quickly become enthusiastic about converting their current film project and throwing themselves out into the talkie scene full-force... until Lina opens her mouth:
R.F.: Don, it'll be a sensation! 'Lamont and Lockwood: they talk!'
Lina: [with a voice to peel paint] Well of course we talk. Don't everybody?
The climax moment of the film is this, awesome, and Heartwarming all at once. Starting with Cosmo happily dancing along to the Singin' in the Rain number, with Don and R.F. gleefully joining in as they dance over to the rope to raise the curtain and badly sing along as they do so. The final clincher is when, as the audience roars with laughter at the reveal of Kathy behind the curtain, Cosmo runs out on stage and takes over the microphone from her to continue the song while the oblivious Lina is still lip-syncing. One of the best movie endings in history.
The scene where they're trying to get the mike to work. First, they put it in a bush. Then, they put it over her heart. (It catches her heartbeat). Then, they put it on her shoulder, and it finally seems like all is well...
R.F: [walks into the studio and trips over wire] What's this wire doing here? It's dangerous.
[he yanks it, and sends Lina topping over backwards with a startled shriek]
At the end of the famous titular number, a policeman comes up to Don (who has been making a real spectacle of himself, splashing around in the puddles and dancing in the middle of the road) and basically glares him away. Don's sheepish, cheesy little grin is adorableand hilarious.