Not the cleverest insult in this thread!
[sub]Grrr… hamsters ate my post yesterday.[/sub]
Not the fastest server in the cluster.
Not the slickest shark in the cards.
Not the freshest apple in the barrel.
If you are not too bright I might call you " five watt".
If someone does something really stupid I might make a snide remark like " you are my hero".
Not the sharpest tack in the drawer.
Not the stickiest PostIt on the pad.
Not the sharpest quill on the porcupine.
If I point out that this one was in the OP, what crayon does that make you 
?
I does that make me the biggest prick in the thread?
A Foghorn Leghorn quote mentioned in another thread:
“… about as sharp as a sack full of dead mice.”
And some of my personal favorites:
“Kid’s about as sharp as ten pounds of wet leather.”
“Kid’s about as sharp as a bowling ball.”
Some other all time (though off topic) winners:
Someone shot the dots off his dice.
Not all of his dogs are barking.
How about:
…ten pounds of dumb in a five pound bag.
From “the green mile:”
“That boy’s cheese done slid off his cracker”
Not the swiftest crayon in the box.
Not the highest card in the deck
Not the freshest loaf on the shelf
not the deadest zombie in the graveyard…
Not the quickest bunny in the forest.
Not the hardest penis in the Porn.
Not the brightest candle in the menorah.
The gates are down, the lights are flashin’, and the bells are ringin’, but there’s no train a’comin’.
Not the straightest dope on the SDMB.
As dumb as a mailbox.
Yes, I does. 
Not the deepest well in the oilfield.
His elevator doesn’t go all the way to the top.