Well I have to say…even if it’s only a chance to quote Oliver Cromwell…“By the bowels of Christ, bethink yourself you may be wrong.”
Sorry, but I think being smart is all about not being completely certain about your own ideas. There’s always new information.
“Yeah, well, the Jerk Store called, and they’re running out of you!”
Otherwise, I just tend to answer those sorts of questions with “Wibble”. Short, sweet, and totally unconventional. 
[Blackadder] I hate to fly in the face of public opinion. [/Blackadder]
“No, I’m not. I just know a lot of stuff.”
<said in slow and absolutely flat deadpan>
“I am wounded, truly wounded. You cut me to my soul. I don’t know how I can go on. I hang my head in shame. You have discovered the very heart of me and laid me open for all to see. I don’t know how I can bear the humiliation now that your razor-sharp perspicacity has carved out my self-esteem and exposed it to the judgement of a cold, cold world. Your withering wit humbles me, truly…”
See how long you can keep it up.
Well, I slept with your wife!
Oh, sure. I don’t think I’m always right, but at any given time, I think the opinion that I hold is right. How could I do otherwise? That’s what it means to hold an opinion: to think that that opinion is right.
“you said it”
I do hear that a lot from other people. Who am I to argue?
Ah, yes, Blackadder has a way with words! Thanks, Martini Enfield. So I guess you think you’re funny? 
Comliments make me uncomfortable, so I’d respond in the same way, a line I learned from Fraiser: “Thank you, It’s always nice to hear that”
You’re entitled to your own opinion.
Psst, his wife is in a coma.
I always wanted to know what the second “airtight” retort was from George. You know, the one where he gets smacked down, drives off, and turns back around because he’s got a better one.
Ooh, that reminds me of a retort that I’ve never had the nerve to use: “Listen, sister, if I want your opinion, I’ll beat it out of ya.” I got this from an Elvira movie. I’m sure it existed earlier, but I love the way Elvira delivers the line.
My response to “So, you think you’re so smart!” is “I hope so, otherwise, all that time I spent learning stuff was a waste of time.”
My husband sometimes says “You always have to be right!” I tell him. “No, I just hate allowing someone I love to walk around with a head full of misinformation!”
I always liked, “How can I put this delicately? If I want your opinion, I’LL READ YOUR ENTRAILS.”
No, only delusional people “think” they’re smart. I, on the other hand, have proof.
Oh, ah, nope.
Him: “I guess you think you’re smart.”
Me: “Lots of people do. What’s your point?”