“Remember the time he ate my goldfish? And you lied and said I never had goldfish. Then why did I have the bowl Bart? Why did I have the bowl?”
“So cold. So very cold.”
Milhous: You’ve changed, man.
Bart: Sod off.
Milhous: It used to be about the music!
Bart: I said SOD OFF! <throws empty glass bottle at Milhous’ head>
After Bart makes a vow to stay out of trouble, the phone rings. Milhouse is on the other end, and says “Hhhey, Bart, wanna go play with the X-ray machine over at that abandoned hospital?!”
“If they Ayatollah can’t have Bart’s soul, NO ONE can!!!”
“How could this happen? We started out like Romeo and Juliet, but it ended up in tragedy.”
Brilliant and subtle.
“But my mom says I’m cool!”
cuauhtemoc, that is possibly my all time favorite Milhous line. But I think you may have it partly wrong. I think it was “you ate my goldfish” rather than “he ate my goldfish”.
“Everything’s coming up Milhouse!”
Nope, cuauhtemoc had it right. “He” is referring to Santa’s Little Helper, in the Laddie episode.
This quote, incidently, was the first one I thought of when I read the topic…
Milhouse and Bart talking about doing a crazy stunt:
Bart: You mean like they do on Jackass?
Milhouse: Yea, all those disclaimers just make me wanna do it even more!
“This is great! And all I’ve done is enter my name: Thrillhouse.”
WELCOME THRILLHO
When Lisa (I think ) is watching the Mexican dance movie, there’s a Mexican Lisa who’s the heroine and a Mexican Milhouse who lusts after her. After winnning the dance contest, the Mexican Lisa goes off with the Antonio Banderas-ish stud, leaving the rejected Mexican Milhouse to lament:
“Once again, I must sugar my own churro”
Others:
“You can’t see her Bart. She’s with the 'House now”
“This is what it sounds like when doves cry”
I totally agree. That one is my absolute favorite!
“Remember ALF? He’s back! In POG form!”
“There’s plenty of Milhouse to go around!”
Milhouse’s signing of Lisa’s yearbook:
“See you in the car. Best wishes, Milhouse”
Milhouse: “School’s out! Up yours, Krabappel!”
[runs out, while other students sit]
Krabappel: “Well, I’m glad the rest of you remembered that summer vacation starts at the end of the day, not the beginning.”
“He said ‘radical’. I say ‘radical’! That’s the thing that I say. I feel like I’m going to explode here!”
“I’ve said ‘jimminy-jillikers’ so many times, the words have lost all meaning!”
Milhouse: Is this the untimely end of Milhouse?
Milhouse2 from Shelbyville: But Milhouse is my name!
Milhouse: But I thought I was the only one!
Milhouse2: A pain I know all too well.
Milhouse: So this is what it feels like…when doves cry.
“Their hands were everywhere!” (sobbing)
And, it’s not a spoken line, but in the ep where Lisa likes Nelson, Millhouse passes a note from her to Nelson reading Guess who likes you? Nelson turns around and Millhouse is doing the eyebrow thing. That just kills me.
I think Milhouse’s sprinkler impression from the beach house ep is hysterical, but it doesn’t work so well in text… 
Followed by a scene of Milhouse strapped in a gurney and being loaded into an ambulance…