Radical Media has put together a series of the best convention videos of all time, 3 Democratic, 3 GOP. The videos contain analysis, explaining why the speeches worked so well. For the Democrats, there are 2 keynote speeches, Cuomo in '80, and some State Senator in '04, as well as Harry Truman's acceptance speech in '48. Not a bad selection, but I've always been partial to Ted Kennedy's concession speech, also in 1980. (The last time a major candidacy was taken live into a convention). I was in Madison Square Garden for both speeches in 1980, and both men would have been great presidents.
Correction: Cuomo's speech was, of course, in 1984, in San Francisco. (But I was there for that one also...) Thanks to ranger11 for the catching the error.