The first two are classics, and always will be. The fact that last years Doctor Who Christmas special completely ripped off the first film shows how much of an influence it still is.
Personally I love Christopher Reeve's portrayal, spefically the fact that you can always tell that he plays Clark Kent as Superman playing Clark Kent; the shot in Lois' apartment where he takes off his glasses and straightens his posture is something that I don't think has been matched in any superhero film (at least that I've seen) since. And only Keaton's Bruce Wayne comes close. Linked to this is Reeve's specific choice to play Superman not as a "hero", but as a "friend", again something which modern films don't really live up to.
Like others have said though, III had some great moments and ideas, but wasn't a great film. As for IV, even Reeve (who helped write the film) essentially said "the less said about it the better" in his autobiography.