I have seen most of the mainstream movies but I am hoping to discover some hidden gems. Below are the ones I have seen already.

Werewolf
An American Werewolf in London
The Howling
Silver Bullet
The Brotherhood of the Wolf
The Company of Wolves
Cursed
Dog Soldiers
Wolf Man (Chaney)
Wolf



Vampire
Interview with the Vampire
Dracula (Coppola)
The Hunger
Let the Right One In
Cronos
Blade trilogy
Underworld films
From Dusk til Dawn
True Blood
Rabid
Lifeforce
Near Dark
Nosferatu original and remake
Dracula (Lugosi)
Shadow of the Vampire

Underworld...has both Vampires and Werewolves, Good film I thought. If you want a horrible film with both try Van Helsing.

For vampire, check out Fright Night.

I'll cover both for ya: The Monster Squad

The 'Ginger Snaps' stuff is a new take on werewolves.

Paco Plaza's Werewolf Hunter-The Legend of Romasanta with Julian Sands.

This sort of has a vampire, and there are werewolves as well:

Martin
Blacula
The Vampire Lovers
Blood for Dracula
The Addiction
Ganja & Hess
Brides of Dracula
Salem's Lot
The Last Man on Earth
Black Sunday

The vampire films of Jean Rollin (Rape of the Vampire/The Nude Vampire/Shiver of the Vampires/Requiem for a Vampire/Lips of Blood/Two Orphan Vampires/Fiancee of Dracula). These vary in quality depending on taste.

'The Company of Wolves' is another werewolf thing worth seeing.

The BBC miniseries Count Dracula (1977) starring Louis Jordan might be of interest, along with John Badham's Dracula (1979) starring Frank Langella.

One I like a lot is Harry Kumel's Daughters of Darkness (1971). Also Vampyres (1974). As with the aforementioned works of Jean Rollin (and Jess Franco (VAMPYROS LESBOS, etc.), these lesbian vampire/Bathory/Carmilla flicks might require that you be a little "adventurous" in your viewing tastes.

For werewolves, one guy to research is Paul Naschy; he did a lot of Waldemar Daninsky werewolf movies.

For vampires I would suggest 30 Days Of Night. I really enjoyed this movie and have seen it a bunch of times.

Also I would recommend Trick R Treat, that's being released on dvd and blu-ray soon.

^ You might wanna spoiler that recommendation.

John Carpenter's Vampires. And how could you forget the classic, Lost Boys?

Another strong recomendation

Gotta put some Hammer in there, my favorites are:

vampire:

Dracula
Brides of Dracula
Dracula: Price of Darkness
Dracula Has Riven From the Grave
Countess Dracula / Vampire Lovers
Twins of Evil

werewolf:

The Hound of the Baskervilles
Curse of the Werewolf

The 2002 BBC version of The Hound of the Baskervilles was pretty decent too, I think it was shown here on Masterpiece Theater.

Vampires:
I remember liking GRAVE OF THE VAMPIRES (1974), starring William Smith and Michael Pataki, a lot. Smith plays a half-vampire who goes after his vampire father.

Also, there's a good b&w vampire western called CURSE OF THE UNDEAD (1959), starring Eric Fleming (Clint Eastwood's future "Rawhide" co-star) and Michael Pate. Fleming later drowned after diving into the rapids while filming on location in Peru for an aborted TV project called "High Jungle."

Werewolves:

There's some good WOLF MAN-inspired action featuring a talking werewolf (played by Matt Willis) in RETURN OF THE VAMPIRE (1943), starring Bela Lugosi as a sort of modern-day Dracula in wartime London.

Wolfen

which reminds me I have to rent 'The Howling 2' which is I've heard is so insanely bad, its actually entertaining.

Bava's Planet of Vampires is also worth checking out.

Vampire-The Night Stalker

I was debating whether or not to recommend this one. In the end, I decided against it because some would say it technically isn't a werewolf movie. A good movie indeed it is though.

nor is 'Brotherhood of the Wolf' either for that matter, the reveal of 'Wolfen' is noteworthy in that well... they aren't lycanthropes in the traditional sense but the film is remarkable in that it's modern tale of supernatural murders set in New York City, with the addition of an amazing brutal ending that has a standout scene of violence that has to be seen to believed. (think 'The Omen')

Thirst

Here are a few that I love.

The most sad-sack, forlorn werewolf ever.

I always liked Embrace of the Vampire....maybe for other reasons.

A favorite of mine, never gets any love at all, as the lack of mentioning in this thread proves.

La Femme Nikita as a vampire taking out the Italian mobsters. Works for me.