Probably Zombieland. See, up to that point, most of the movies I'd seen were with friends or family out in the suburbs, so the crowds were small and sedate. Oftentimes, we'd be the only ones in the theater, which is usually quite nice.
Zombieland I saw in a crowded theater full of (fairly well behaved) college kids. Not only was the movie excellent, the company was perfect for it--it was like a stadium of people all on the same wavelength, laughing and gagging and falling dead quiet at all the same moments. It was lovely.