I assume you mean a real martini, right? Gin, vermouth, olive? If so, it's not like anywhere is going to serve you a bad one, but the lounges at Shula's and Bluezoo are both nice spots to have a drink. Bluezoo in particular has excellent bartenders and a nice atmosphere.
I don't drink a lot of martinis at Disney but I do like Le Cellier's Ice wine martini - extra dirty, extra olives. It is made with Canadian Ice Wine, vodka, and olives/juice. It isn't on the menu any longer so you have to ask and see if the bartender knows how to make it.
Thanks! This post really hit the spot - gin, vermouth, and olives are definitely the basics - but you really upped it a level when you mentioned Hendrick's, a long-time friend of my husband Thanks everyone for the great suggestions! We'll also be checking out the blue-cheese stuffed olive martinis mentioned too, because those are near and dear to MY heart