One that sticks in my mind is when Flik finds out that he brought circus bugs to the island after he had been convinced that they proclaimed themselves to be warrior bugs and he says “This my friends is False Advertising!”. I found that line very funny for some reason.
And the “Turn your butt off!” line is just classic. 
“My Children’s, Children will be stopped in the street and people will go “look there goes the spawn of Flik, The Loser!””
-Flik
LOL, yeah I love that line too! 
I have A LOT of favorite quotes from ABL. Here are some that I can think of:
Slim: What’s the point of going out there? They’ll only laugh at me.
P.T. Flea: That’s because you’re a clown!
Slim: No, it’s because I’m a prop. You always cast me as the broom, the pole, the stick… a splinter!
P.T. Flea: You’re a walking stick. It’s funny! Now go!
Slim: You parasite.
Slim: [dryly] Tra-la-la-la-la. Spring is in the air, and I’m a flower with nothing interesting to say.
Mosquito #1: Harry, no! Don’t look at the light!
Mosquito#2: I-can’t-help-it. It’s-so-beautiful.
Rosie: And that’s how my twelfth husband died. So, now I’m a widow. I mean, I’ve always been a black widow, but now I’m a black widow widow.
Slim: Francis, you’re making the maggots cry.
Slim: [whispers] Francis, your boyfriends from the circus are here!
Francis: Hey, turn your butt off!
Heimlich: I’m a cute little bumblebee!
Hopper: Are you saying I’m stupid?
Princess Atta: [faintly] No
Hopper: Do I look stupid to you?
Dot: But it’s a rock.
Flik: I know it’s a rock! Don’t you think I know a rock when I see a rock? I’ve spent a lot of time around rocks!
Flik: Your Highness! The warriors have called a secret meeting to make plans for circus, uh, circumventing the oncoming horde, so they can trapeze - trap them with ease!
Princess Atta: Well, shouldn’t I come, too?
Flik: No! Ha ha ha! Sorry. No, no, see it’s classified in the DMZ, gotta go ASAP, you know, strictly BYOB. Bye!
P.T. Flea: FLAMING DEATH!!!
Heimlich: I’m a beauuuutiful butterfly!
Slim: That’s no way to speak to a lady.
Francis: I heard that, you twig.
Slim: [stuck among branches] Francis! Francis, I’m stuck!
Francis: Where are you?
Slim: I’m over here!
Francis: Where?
Slim: Here!
[irritated]
Slim: I’m the only stick with eyeballs!
Worker Ant #1: I’m lost! Where’s the line? It just went away. What do I do? What do I do?
Worker Ant #2: Help!
Worker Ant #3: We’ll be stuck here forever!
Mr. Soil: Do not panic, do not panic. We are trained professionals. Now, stay calm. We are going around the leaf.
Worker Ant #1: Around the leaf. I-I-I don’t think we can do that.
Mr. Soil: Oh, nonsense. This is nothing compared to the twig of '93.
There were a lot of great quotes from the bloopers too. Though my favorite ones are when Atta laughs at Hopper and when Flik quotes Woody from Toy Story and everybody is cracking up at the background. 
I never understood what BYOB meant. When you’re younger a lot of things go way over your head, but this is one you’d think I’d get by now. That’s intelligent humor for sure.
That was Buzz that Flik was quoting, but they showed Woody using the clapper.
LOL, oops. I did meant to say Buzz. 
My mistake.
EDIT; From the bloopers, the “Princess Abba”, “Put that thing in my car.” and Slim’s “I think I swallowed a bug.” ones are pretty amusing, to say the least.
[b]DON’T WORRY! THE COLONY IS IN GOOD HANDS!
Bye now.[/b]
Boy ant: When I grow up, I want to be a Preying Mantis!
Other boy ant: I want to be a Stick Bug!
First boy out: I can’t choose, they’re all so cool!
Heimlich: I am a cute little bumblebee! Here I come! Slow down, you flowers! sees fly kid with candy corn Oooo, candy corn! Here! LET ME HELP YOU TO FINISH IT!
Manny: Yet again it is up to me to rescue the performance! Gyspy, come!
(Gypsy is humming; something crashes in the background and Manny screams)
Gypsy: The stage is the other way, dear
Manny: (completely unfazed) Yes, of course!
My Absolute Favorite, simple, but just hilarious:
“Your dad’s right, he’s gonna die.”
Hopper to Atta: “Stupid? Do I look stupid, to you?”
Waitress in Bug City: “Hey, who ordered a Poo Poo Platter?”
[Lots of flies attack the plate]
Heimlich: “I’m a cute little bumblebee!” and “I’m a beautiful butterfly!”
Fly1: “No, don’t look at the light!”
Fly2: “I can’t help it, it’s so beautiful!”
Dissin’ Fly: [To Francis] “Hey, cutie, wanna pollinate with a real bug?”
That’s all I can think of for now.
I think everybody has already quoted all of my favorite lines from A Bug’s Life, so I don’t think it would make any sense for me to post them again… 
But I honestly have to say that the one line that Flick says in the beginning is what I personally think the film is all about:
“You might not think that you can do much now, but that’s just because you’re not a tree yet. You still have to give yourself some time…you’re still a seed.”
It’s amazing how that line really carried through the entire film, not just for Flick, but for all the characters: the Circus Bugs and how they were depressed that they would never have an audience, Dot trying to fly and rise above everyone taunting her that she’s so little, Atta always worrying that she’ll never become the queen that she needs to be for her colony…and of course, Flick and his dreams to save his people from the clutches of Hopper.
It’s just one of the amazing qualities of A Bug’s Life…which is why it will ALWAYS be one of my top Pixar favorites!
I’m not quite sure. Probably this:
Flik: They can’t know the truth! The truth you see is bad! I will be granted with this mistake for the rest of my life! My childrens’ children will walk down the street, and people will point and say “Look! There goes the spawn of Flik the Loser!”
“From up here you all look like little ants! Auf wiedersehen!” -Hiemlech
(I hope I spelled all that right! 
)
“But it’s a rock.”
“I know it’s a rock! You don’t think I know a rock when I see a rock!!! I spend a lot of time around rocks!!!”
“Why am I even talking to you? You’re not the Queen; you don’t even smell like the Queen.” - Hopper X3
Oh yeah, here’s another one I like:
Hopper: I swear, if I hadn’t promised Mother on her deathbed that I wouldn’t kill you, I would kill you!
“And believe me, no one appreciates that more than I do. Aaah! Remember ma!”
“It wasn’t even my idea! It was Axel and Loco’s! They talked fancy to me!!! I GOT CONFUUUSED!”
Hooray for Molt love.
For the colony and oppressed ants everywhere!!!
Good one!
“My children’s children will walk down the street, and people will point and say, there goes the spawn of Flik, the loser!”
That’s not word for word, but it was something like that. Defintely one of my favorite lines from the movie!