There are many convincing arguments to the contrary here, the White Sox softball uniforms foremost among them, but I have to go with the flaming Porta-Potty that is the Expos/Nats franchise:
-Olympic Stadium. An absolute hideous playing surface, likely to have been at least partially responsible for the early demises of several great players;
-Nationals Park, aesthetically pleasing, but a mid-June day game there is like watching baseball on the surface of the sun;
-Expos were on the verge of winning the NL East and probably the best team in baseball when MLB went on strike, derailing their one serious bid for a World Series. Not their fault, but another chapter in a horrible history;
-the one other year when they won their division, their season was also marred by a player strike, suggesting they may have also been cursed (I'd have to consult a medium for confirmation);
-sold them off for parts, and taken over by MLB, they then spent portions of a couple of years playing home games at a Double-A park in Puerto Rico;
-they also had no television or radio contracts toward the end of their time in Montreal (which I'd forgotten until I fact-checked on Wikipedia);
-eventually became NATSTOWN, or the Natinals. Inept and awful at the start, they became decent largely through drafting talent that was only available due to their horrible finishes;
-injury and mismanagement continue to plague the franchise to this day, resulting in epic collapses and despondency among a dismal fan base.
Added to all that, the Nationals just feel soulless, in a soulless city. The only way to reverse the suck is to re-build Jarry Park and let them go back to Montreal.