The dreams aren't as surreal as in later installments, but the original A Nightmare on Elm St. still holds genuine tension and although Krueger would later become a punchline, it's not hard to see why he became such a fascinating figure off the back of the film. The movie benefits greatly from Craven's still low-budget sensibilities, which grants the picture a gritty authenticity that all of its sequels lacked. And unlike 90% of all slashers, you actually care about the characters that are being hunted.
Also, this series has the best poster art of any horror franchise I can think of. Evocative and memorable.