My favourite has to be the first. The idea was new and it was the first and best version of Freddy, in all senses. If you notice, in the first movie they keep Freddy in the dark/shadows pretty often. It makes it a little harder to distingush his features and have a distinct idea of what he looked like. That face was the most scary.

I think they just had to change the make up due to the character spending more and more time in the lime light and it had to stand up to being more visible and more durable as Robert England had more scenes to be in.