WHAT'S THE MOST POPULAR apparel at the COOP these days? According to David Sullivan, the store's working manager, both students and tourists prefer a gray Harvard sweatshirt with raised crimson lettering. (Price:$ 49). For some, the sweatshirt sells a dream, the closest they will ever get to living the Harvard experience. For others, it provides a sense of satisfaction, an opportunity to ostentatiously display their superiority. Either way, there's a lot of sweatshirt buying going on. Pointing to a pile near the front of the store, Sullivan explains, "With students, buying Harvard apparel is a tell-tale sign that you're a freshman. They buy everything, but like the tourists, they most often purchase these." Apparently, most consumers choose the Gear version over its Champion counterparts. "They're cheaper, and they're of better quality," David says. And they look exactly the same. (Well, almost.) The most popular size? XX-large, for the shopper who just can't get enough HARVARD.