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How bout "I'm a doosh"? Any other catchphrase will ultimately mean the same thing.

"I am a knight of the order of the burning purple! Surrender now, for you are sure to lose!" Can start off the game on the right foot.

How about, "Now that I've finished with you, I go after your family."?

For winning:

http://www.youtube.com/watch?v=M5QGkOGZubQ

How saying NOTHING- Sore losers and braggarts are certainly not much different

"To be the man, You got to beat the man..WOOOOOOOO!!!"

What you're looking for is simple: "BUuRRRRPPPP!"

That's it. Win or lose.

I'd personally go for, "That's the way the cookie crumbles" or something awesome like "I'm the Juggernaut... madam"

Rude and no imagination?  What a tortured existence that must be, watching your feeble insults bounce harmlessly off of intended victims.  The hallmark of a real jerk is being able to generate biting banter on the fly as the situation dictates.

"that was so exciting"

What about farting??? Can you fart in text?

Maybe something like

 (o) brraaaaappppppp!!!!!!!!!!!!!

I sometimes write "I fly on the wings of victory"

"Game invalid.  Transcription error on move number 3.  Please contact support."

Good game or its variants (gg, etc.) is all you need at the end while Good luck (gl, etc.) and all its variants are for the beginning.