When it comes to classic genre films which are 'essential viewing', there isn't a definitive list, but, in 1999, Empire released a Special Collector's Edition supplement called The Greatest Horror Films Ever, which is a really good 'beginners guide' to horror. In it, there are 50 films in different categories of films, which I'll list here.

Creature Features
Frankenstein
Nosferatu
Dracula (1931)
The Mummy
The Bride of Frankenstein
Cat People...