If you're going to have a calculator watch, why trifle? Why bother with something that only performs basic arithmetic? You're trying to make a statement, right: that math is important to you. You should be able to do basic operations in your head. If you're going to wear a calculator on your arm, please wear a badass one.
I'm not sure if the 1983 Casio CFX-200 is the best balance of function and I-could-actually-wear-this-thing, but it's the most wonderful usability trainwreck I can find. 23 buttons! Many of which are multifunction buttons. Surely this is the Dodge Viper of calculator watches, in which misuse will almost certainly lead to death. Got a better one? I would loooooove to know about it. For now, you can check this baby out on eBay. Auction's going on now. [eBay via Oobject]