Here's a few of mine:
Handgun Control Inc. (or any of their ilk) -- they'd disarm everyone except the police if they could
American Medical Association -- by artificially restricting the supply of doctors, few other unions have done more harm to U.S. citizens
PETA/Animal Liberation Front -- bomb and harass scientists working to cure aging and disease
American Trial Lawyer's Association -- macro parasites par excellence
Federation for American Immigration Reform -- don't like immigrants; wants to control who you love, hire, or otherwise associate with
RIAA/MPAA -- their state-granted monopoly stunts new distribution channels, and the development of new works; much of our intellectual heritage lies rotting in their vaults
Note, I've only included those that have a measure of influence/power. While vile, organizations like the KKK, American Communist Party, etc. have little influence at the moment, and therefore, don't represent much of a threat.
Also, I'd prefer to avoid debating whether any of these groups should be on my list. I'm just curious about who is on your list.