Just read V for Vendetta for the first time. Really wish I'd read it before seeing the movie years back. I made a point of reading Watchmen before the movie came out, but when V was in theaters I'd never even heard the name Alan Moore before.
Now buy:
The Killing Joke - Moore's 'definitive' Joker story
Miracleman 1-16 - Moore's incredible, and incredibly intense, superhero story
Then before approaching others, to cleanse your palate, a fun sort-of tribute to Doc Savage:
Tom Strong [published by ABC comics]
Then, if you want some really unusual stuff:
Saga of the Swamp Thing issues 20-38,
Swamp Thing 39-64 and
Swamp Thing Annual 2 - Moore takes the character into uncharted areas [this was the series that got him 'noticed' in America]
Promethea - for a very unusual step into, well, just about anything and everything (starting with the meaning of creativity). This one doesn't stay in standard story form, and the depth of what he approaches is so intense it can require some effort at times.
[I'm a big Alan Moore fan.]