The first time I saw An American Werewolf in London was at my 14th birthday/slumber party. A friend had brought the video for us all to watch, but unfortunately when we got to the nightmare with the Nazi zombies I got rather freaked out and ended up being sick in the kitchen. Not exactly my finest hour (in my defense, I had a rather strange phobia of people getting their throats slit at the time, and was under a lot of stress from other sources as well). Thankfully I'm over all that now (at least I'd hope I'd be after 22 years) and so I can properly appreciate the entire movie as it was meant to be appreciated.
Two Americans, David and Jack, are touring Europe and have hit the England portion of their tour. Arriving on the Yorkshire Moors, they briefly visit a pub called The Slaughtered Lamb where the locals are only slightly stranger than usual for isolated British village pubs (trust me on this), before setting off again to get attacked by a werewolf when they get lost on the Moors. Jack is killed but David is only injured before the locals arrive to kill the werewolf, and so David is taken to hospital in London to recover. It's there that he learns from his undead friend Jack (that only he can see) that he has inherited the curse of the werewolf and will become one on the next full moon, unless he kills himself. Has David just snapped as a result of the attack and death of his friend, or has he really become a werewolf?
Director John Landis apparently got the idea for An American Werewolf in London all the way back in 1970 while working on another movie on location in Yugoslavia, when he came across a Roma funeral - the deceased was being buried feet-first and wrapped in garlic to stop him from rising from the dead. It took him nearly a whole decade before he had the funds to actually make the movie, but I think we can all agree that it was all worth the wait. One of the main reasons for this is the Oscar-winning work of special makeup effects maestro Rick Baker, whose work on the werewolf transformation scene has become a benchmark against which all other werewolf transformations now get measured (his work on the undead Jack and later on the other werewolf victims is also spectacular, of course).
Another reason why An American Werewolf in London is so beloved and has stood the test of time (and the fickleness of horror fans) is its humour. It took a long time for Landis to get the funding for the film because many financiers thought that the film would be too funny and not horrific enough to be a 'proper' horror film - producers even wanted him to cast Dan Akyroyd and John Belushi as David and Jack. Now I love The Blues Brothers as much as the next person with taste, but I think we can all say that the film wouldn't have been anywhere near as good if that had been the way the casting went. However, the humour that is there makes the characters in the film seem much more sympathetic - including the various victims of the werewolf, some of whom helpfully suggest ways to commit suicide as David hides in a porn theatre in Soho, despairing of his situation.
(Amusingly, many of the audience when the film came out also believed, because of John Landis' previous work on films like National Lampoon's Animal House, that the film would be an outright comedy, and a great many of them ended up fleeing the cinema is horror when they actually saw all the blood and gore on screen. I like to think that a lot of them were broken by the Nazi zombie nightmare just like me.)
Now I will freely admit that, out of all the classic horror movie monsters, werewolves tend to be the ones that impress me the least, but An American Werewolf in London is one of the few exceptions to that. I think it's because it focuses less on the mythology of werewolves or the whole "humans have deep-down animalistic tendencies and werewolves are the psychological release of these subconscious desires" theme that often comes up in werewolf films, and is much more about the horror of becoming a werewolf, especially when the protagonist is an everyman that the audience can identify with. And there's no actual villain, either, just people with varying degrees of bad luck. So I most definitely have to recommend An American Werewolf in London, if for no other reason than any werewolf movie that I actually enjoy has to be a good one.