Well, this should be fun.
1. The Quatermass Xperiment - Arguably, the first true "Hammer Horror" flick, it was also a great film in it's own right, and set up Hammer productions for a great run. In order not to be cheesy, let me just say that I loved the sequel, too. Interestingly enough, the film is vaguely Lovecraftian.
2. Horror of Frankenstein - Ungodly goofy, and kinda intended to be so. David Prowse (who would go on to be everyone's favorite bad guy, Darth Vader) was the monster. The film was a remake of Hammer's first Frankenstein flick, and they opted to have some fun with it, making it arrogantly odd. Well worth seeing, repeatedly.
3. One Million Years B.C. - OK, not a horror film, but a high point of Hammer's movies. Raquel Welch in a fur bikini? Harryhausen doing the animation? How can you go wrong?
4. The Phantom of the Opera - I have a soft spot for most of the Phantom movies, over the decades, as I'm a huge fan of Leroux's original novel (I strongly dislike the Webber musical and the movie of it, though). Herbert Lom turned in a great performance here as the Phantom. That is almost too bad, as he went to the Pink Panther films right after this, and that's pretty much how he's now remembered.
5. I couldn't choose one of these and not the others, so I'll lump the entire Karnstein trilogy into one: The Vampire Lovers, Lust for a Vampire, and Twins of Evil. The series was based around Le Fanu's vampire story Carmilla, with a lot less story and a lot more breasts thrown in. Yes, they're great.