Movie: American Beauty. People had hyped it up to me as one of the best movies ever, and when that happens I've learned to expect disappointment. I went in expecting just a good drama that was nothing spectacular, but nope, it's a movie that would very easily make my list of top 10 movies of all time, and probably in the top 5 at that.
Show: Steins;Gate (it's an anime, for the non-weebs out there). It was one of those anime that I saw brought up a lot in the last few years whenever people talked about anime, and it was another one that lots of people billed as one of the best of all time. I first watched the vastly inferior and not really a prequel Chaos;Head, because I thought that since it was set in the same universe then they were obviously related, but as it turned out that was not the case. Anyway, I finished that series and was really leery about going on to Steins;Gate because it hadn't been that great and series usually decay in quality over time, but I saw that it was made by a completely different studio so I decided to give it a chance... and now it's one of my favorite anime and would easily take a high spot if I were to make a list my top anime of all time.
Also, I would have said The Wire if Empress Wen hadn't already covered it, so I definitely second that as one that deserves the hype.