Via Volokh Conspiracy, here's a pretty amusing site that is an index of what the author refers to as "idiot legal arguments" and case citations in which courts promptly reject and demean such arguments. This is actually pretty handy if you're a lawyer up against someone making one of these arguments, or even if you just feel like arguing with those anti-tax nuts who think the income tax is illegal. I'd be hard pressed to pick my favorite, but I think the one about the "phantom " 13th Amendment that outlaws titles of nobility, including the titles of members of Congress, thus somehow stripping of their power to make laws that are being enforced against the offerer of this argument, is so outlandish as to be endearing.* Please note your favorite in comments.
*Though the argument that by virtue of the 1792 Coinage Act Federal paper currency is not taxable is also pretty amusing.