If you liked Suspiria, I recommend the sequel Inferno. It's a very different sort of story, but it's as visually rich and absorbing as Suspiria. I also will admit, Mater Tenebrarum is my favorite of the Three Mothers.
First Period is a hilarious, brilliant comedy with a distinctive aesthetic, an amazing cast, and a lot of great things to say. I didn't think I'd enjoy it as much as I did, and then I watched it again, and again, and again. It's such a clever script and has so many quotable lines. My friends and I are always quoting this film!
Prince of Darkness is an unconventional approach to horror, mixing in science-fiction and an unusually mature cast. "This is not a dream...this is a transmission" will haunt your memories!
Horror Express is a strange and horrific classic, featuring iconic performances by Christopher Lee and Peter Cushing...and be sure to watch for Telly Savalas later! A really superb film that does an excellent job with its choice of setting.
Vampire Circus is a very different approach to the vampire story, and it's probably my favorite Hammer vampire film overall. It dares to tell a very different story in a very different way, and it mostly succeeds in what it sets out to do. I fell in love with this one the first time I saw it, and I've always loved it.
On this same vein, I have always loved the film The Monster Club, which features not only solid (and unsettling) segments, but a wonderful linking story with Vincent Price and John Carradine in rare form. Also, uniquely for the genre, musical numbers! Some of which you'll probably recognize!
Vincent Price also features in the excellent films The Abominable Dr. Phibes and Dr. Phibes Strikes Again, both of which are legitimately scary and darkly hilarious. He's such a compelling villain that he really becomes something of an antihero, but both of these films feature some really imposing scares that are done with class and style, and they even surprised me, which isn't easy to do!
Last but not least of all, Screamtime is an anthology film from the early 80s and features three legitimately good segments, but the standout segment is "Dreamhouse". Please don't spoil it for yourself -- watching it cold will blow you away!