Run out and get The Watchmen right now. Actually, almost anything by Alan Moore is worth the price of admission.

I'm more into the collected works stuff rather than standalone graphic novels, so I'd throw Frank Miller's various Sin City series stuff in there along with Garth Ennis' run on Hellblazer as well.

Anything by Mark Millar or Brian Michael Bendis. A recent favorite is Ultimates Vol. 1 - found here at Amazon.com

I'm not real into them, but the GF really loved "Blankets" by Craig Thompson.

And what's your GF's name again? Anyhoo, I'm watching the Watchmen. And I'm way into Clowes, too.

I'd give 100 Bullets a try. Azzarello and Risso are amazing. Plus the story arc is great. Amazing characters, premise/plot is engaging. Pick up the first trade (there are 11 so far) that way you won't get stuck in that comic rut of having to wait for subsequent story flashes every 30 days or so. Also, look at Brubaker's work (esp. Sleeper).

Watchmen, definitely. I also recommend The Sandman, by Neil Gaiman.

I never was able to get into Sandman for some reason ...

I'd also recommend Scud the Disposable Assassin if you can manage to track down some copies ... fun pop-culture snack food.

Wow... we all forgot to mention Batman: Dark Knight Returns, which might be the most awesome graphic novel of all time. It certainly made Batman cool again. More info here: Batman: Dark Knight Returns