I would say Onlyinboards.com and if you do, it is worth paying for the paid listing to get more pics and better placement. Craigslist is the other spot I would use.
I just sold my MC X15 and I listed it with OnlyInboards and Boattrader. I think all of the responses I received were from boattrader. When I look for boats, I go to OIB first though...