EDIT 2: I will update this a little and spread it around.
EDIT: I get a lot of good material so I may just be updating this regularly.
TRIGGER WARNING FOR AWESOME!
"Get a life."
This one would only work if you were trying to insult a dead person. May work on zombies and emo kids.
"Yeah, you keep thinking that."
This will work on zombies. It makes them feel like shit so it could count as a blow. They have no brains. This is why they want yours.
"Learn to spell."
This is good for when you are confronting a really bad wizard or when you have nothing better to say. I reccomend it in moderation.
"You're just a hater."
It's always valid. Always. Everyone hates something. If they throw it back at you just say they're a bigger one. They will be no doubt speechless at your wit.
It can be just as effective as "Well, you're a bigger one!" Be prepared to have a much longer conversation when you use that though. That doesn't matter. You are brilliant.
Always effective for shooing.
"Insert offensive one word insult here."
Words like "nigger", "cunt", and "faggot" work best for this one. It doesn't always have to be one-worded. It usually does the job good though.
"You're like Hitler!"
This works on trolls especially. Bullies deserve to be called for what they are.
Great for when the responder has mommy issues! Don't forget to laugh when that meanie cries! 'Your dad' and 'your mother's sister's hampster's etc.' is okay but remember, they're the tryhard not you.
"You're kind of a bitch."
This works if you still want to be the good guy but want to take a jab at someone else. Any insulting word can replace bitch. I reccomend something more creative like "sweaty nazi cactus". Semi-indirect attacks always work!
"You're just jealous/U jelly?/I'm just better than you."
This works every time especially with bullies and insecure, morally outraged women folk. Repeat it as much as you like. Use it heavily if you want to. Variants of this are "You have no friends.", "You have no life.", etc.
"Whatever/.../I don't care/Meh./I don't give a fuck."
This needs to be reiterated often too. Heaven forbid your opponent get confused and make the off assumption that you give a shit! I mean, seriously, what's wrong with them? You are not even pooping. Besides that, despite responding you should not even bother. You can still make long extremely thought out and genuine replies without giving a fuck. It's magic...the magic of being right. Besides, anyone who is against your ideals/opinions is worthless and a lost cause
"Your penis is small/your vagina is huge!"
Anatomy assumptions based on how you perceive the person to act are pretty nifty. Their response will prove it's true every single time.
"Insert Excess Emotes of Joy here."
You are getting a good laugh about this. Even if it seems like you are dying inside with every response. This is key. Use this as much as possible. If you are accused of spamming you did it right!
Super effective on those with debilitating diseases. It is fairly effective on icky people too. A healthy person may slap you with broccoli for that though(after jumping jacks though of course).
If all else fails, troll back. But it is okay when you do it. It is for a good cause...for mankind in fact!
"Like you're any better!"
The person criticizing you is always a hypocrite whether they do what you are doing or not. Another explanation for this is that they hate you and hating you is like hating themselves. This is fact.
"I was joking./Just kidding!/I was trolling this whole time./You fell for it!"
Not only do you not give an ounce of a fuck but you were kidding FOR THE LULZ! It is not backpedaling it was just the super secret yet totally obvious intent all along. Use this especially when someone claims to be trolling you. They will sure be in for a surprise!
You won the battle but is the war against evil over with? Yeah, probably. You even said it was. Now gloat like you never have in your life! Another variant that works if they keep responding is "I am done with you." or "Good Day/Farewell, sir or madam." Try and end the conversation but always reply when they reply back. You already won and the battle is basically over but...hey, you could win some more perhaps? Nothing is wrong with adding another notch on your belt. If they say it back say "Whatever helps you sleep at night."
Now that I have armed you with knowledge be sure to use it wisely.