OK, "greatest teen comedy ever made" really isn't setting the bar all that high, but this really is a wonderful film. I almost docked it a star for being at least partially responsible for a lot of copycat garbage that followed, but that would be punishing it unfairly. It deserved the tons of money it made.
Many of the other reviews on this site have rehashed the plot(s) and the fact that it introduced an amazing array of acting talent to the world. What I think is most impressive however, is the honesty and poignancy this film delivers while being hysterically funny. The relationship between Brad and his sister is incredibly touching. The characters are real, three-dimensional people (yes even Spicoli; I knew someone almost exactly like him in school and I'll bet you did too). It is beautifully written, directed, and acted. It is also one of the most quote-worthy films this side of Tarantino ("All I need are some tasty waves, a cool buzz, and I'm fine").
On a final note, I disagree with the Amazon reviewer's assessment that Cameron Crowe is an overrated director. "Say Anything" is probably the best teen movie made since this one, and "Almost Famous" is a stone cold masterpiece, IMHO.