your words are about as dense as your head, I'm surprised you haven't floated away yet."
Or
I respectfully decline the invitation to join your hallucenation.
Or
"IF I cared, I'd draw you a map of your ass with an X marking the spot where your head is buried."
Or
Go the Southern Charm route - you can say the meanest most vicious things in the world to a person - as long as at the end of it, you smile sweetly and say, "Bless your heart!"
And my favorites are:
Some people cause happiness wherever they go; you whenever you go.
"I can explain it to you, but I can't comprehended it for you."