I've found that the best your mom jokes are when you just respond to someone with "your mom". It's general enough to be insulting, but strange enough to be vague and confusing. Also, "Your mom is like a brick; she's dirty, heavy, and gets laid by Mexicans".
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I've got another: your FACE!
/obligatory
Another slow day at work. Another day of reading comic archives.
Edit: So I didn't want to post another one so soon but it was just too good to pass up.