I've loved both of Nolan's Batman films, but the only older one I've seen is the 1989 original. I had the novelization of Batman Forever on tape as a kid (since I'm visually impaired), but I never actually saw the movie. I kind of want to see it now out of curiosity, but its reputation seems to be very negative. Why?
Batman Forever, though not as bad as
Batman & Robin, has many of the same flaws. Joel Schumacher's Batfilms were campy, shallow, unfunny attempts at comedy merged very inconsistently with attempts at melodrama. They reduced the characters to flashy caricatures and jokes, and elevated the production design to an insane level of garishness.
Here's a page that humorously deconstructs and critiques
Batman Forever in thorough detail:
http://www.agonybooth.com/recaps/Batman_Forever_1995.aspx
Actually the 1989 Tim Burton film is far from the "original." The first filmic appearances of Batman were in the 1943 serial
Batman with Lewis Wilson and Douglas Croft and the 1949 serial
Batman and Robin with Robert Lowery and John Duncan. The first is a disturbingly racist WWII propaganda piece with Batman as a government agent battling a supposedly Japanese prince who sounds more like a Brooklyn gangster and has a name that isn't even phonetically possible in Japanese. The second is more innocuous, but about par for the course for '40s adventure serials, as Batman, Robin, and Commissioner Gordon deal with the high-tech villain The Wizard and his remote-control and disintegrator rays (which somehow become an invisibility ray when combined).
The first feature-length Batman movie was the 1966 film spinoff of the
Batman sitcom with Adam West and Burt Ward. It's a different kind of Batman, but I consider it a comedy classic.
The best Batman movies are in animation, though. There are four movies (one theatrical, three direct-to-video) in the continuity of the Warner Bros.
Batman: The Animated Seriesand its spinoffs, the best of which are
Batman: Mask of the Phantasm and
Batman Beyond: Return of the Joker. The other two B:TAS films,
Batman & Mr. Freeze: SubZero and
Batman: Mystery of the Batwoman, are not as good, but they're a damn sight better than the Schumacher films. Batman has a minor but impressive presence in another Warner Bros. DVD movie (in an unrelated continuity),
Justice League: The New Frontier. There's also
Batman: Gotham Knight, a DVD release containing six
anime shorts from different Japanese directors, which are more or less in the Nolan film continuity and take place between
Batman Begins and
The Dark Knight (although there are a couple of significant inconsistencies between the DVD and TDK). If you like
anime, it's definitely worth checking out.