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Check out Argento protege Michele Soavi's films as well. Cemetery Man is up there with Argento's best.

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What's already been said is pretty much accurate - except, I liked SLEEPLESS - I thought it was pretty good for a late era Argento.
Some things to expect:
Not sure how much experience you have with Italian horror, but know that the dubbing is pretty laughable, but you'll get used to it.
Know that the music is HORRIBLE! Argento in particular had/has a horrendous taste in music, and time and time again he will build up to what should be a strongly disturbing murder sequence, and will then drown it out with 70s/80s death metal music that will leave you reeling with laughter rather than horror.
Also, expect that each movie will completely implode on itself in the last five minutes when the killer and their ridiculous motivation is revealed (usually that they saw a murder as a child and therefore became a psychopathic murderer as adults)
Ones I'd save for last TENEBRE, DEEP RED, and SUSPIRIA. I think SUSPIRIA is always going to be his master work - one of the few where the music really aids the movie rather than distracting from it - and just oh-so-beautifully shot every last scene is a painting in motion.
Oh, one other thing - if it has his daughter in it, don't watch it

Argento's must see movies: (in my own opinion)
Suspiria
Deep Red
Tenebre
Bird with the Crystal Plumage
Stendhal Syndrome
Phenomena
Inferno
Trauma
Sleepless

Thanks for that buddy now it's just a matter of finding the time and finding the movies haha, Ive been wanting to see suspiria for a long time got the dvd at home but never watched it before i left for australia (hopefully worth the wait haha) also havn't come across any place that sells it or rents it here. I will keep a look out for the others aswell.
"He who makes a beast of himself gets rid of the pain of being a man" - Dr Johnson

Essential Argento movie viewing:

1. Suspiria
2. Deep Red
3. Inferno
4. Tenebre
5. Opera
6. Bird with the Crystal Plumage
7. Phenomena
8. Two Evil Eyes
9. Stendhal Syndrome
10. Four Flies on Grey Velvet
    The Rest:
11. Mother of Tears (probably rank it higher if you want to complete Three Mothers Trilogy)
12. Cat O Nine Tails
13. Trauma
14. Sleepless
15. Door Into Darkness
16. Jenifer
17. Pelts
18. Do You Like Hitchcock?
19. Giallo
20. The Card Player
21. Phantom of the Opera (forgot all about this one earlier, his WORST film in my opinion)
    Never seen Five Days in Milan. I'd say the top 10, and even Mother of Tears are the ones to see. I love Italian horror films, I have 145-150 Italian horror movies on dvd haha.

Thank You guigui-paul
"He who makes a beast of himself gets rid of the pain of being a man" - Dr Johnson

PHENOMENA, OPERA, TENEBRE, DEEP RED, THE BIRD WITH THE CRYSTAL PLUMAGE and THE STENDHAL SYNDROME are very recommended!
Argento is one of my very favourite directors and i tend to like even his lesser works but the titles i've recommended to you above are a solid start for a newcomer of italian horror, i guess!
http://www.myspace.com/guillaumep
http://darioargentofr.blogspot.com/