http://www.dccomics.com/sites/essential30/
From this list:
All Frank Miller Batman stuff (I've only read Year One and DKR, but both are excellent -- but then again, I'm of the mind that Frank Miller can do no wrong, including Ronin and Robocop).
Grant Morrison & Dave McKean, Arkham Asylum is especially fun because it comes with the script, so you can see just how much Dave McKean contributed to it.
Neil Gaiman, Sandman the entire series is probably my favorite comic ever, I've been getting the Absolute editions.
I've enjoyed what I've read of Ex Machina (which is up to vol. 5 or 6 of the trades), it's a really good anti-super-hero comic.
I just bought Watchmen.
Y: The Last Man is amazing, and not just because of all the lesbians.