Name something that geeks are passionate about, and chances are good it is available on a T-shirt. Add that to the fact that geeks, even more so than most of mainstream America, are dedicated to casual wear and comfort, and the primacy of the T-shirt should come as no surprise. Combining form and function in one soft, cotton (or poly-cotton blend) embrace, the geek T-shirt is a damn near perfect garment.
Some will argue that the geek's love of the T-shirt is simply more evidence that we are overgrown children, refusing to grow up and accept adult wardrobes. Those people can choke on their stupid fucking polo shirts and button-down short-sleeve business casual bullshit. Wearing that stuff simply announces, "Hey, I work in an office and thus am obligated to dress this way." I'm not sure how this capitulation to corporate culture is supposed to be an improvement on the average geek's vintage Star Wars shirt, which at least says, "I like what I like and I'm not afraid to tell the world about it."
For me, that means zombie shirts. I have a fair number of non-zombie shirts, including everything from a Magic: The Gathering guild T-shirt (Izzet for life!) to an officially licensed Denver Broncos shirt, but by and large if you run into me out and about, I'll be sporting a zombie shirt of one sort or another. I have zombie shirts tied to specific films, like this kick-ass shirt that riffs on Return of the Living Dead:I even have my very own custom-designed shirt to pimp my zombie website: