Gunslinger Grils, that series didn't seem to me as striking or have as much of an intrigue as Noir. I made it till the 2,d or 3rd episode and I completely disconnected from that series.
Noir was one of the first anime series I saw back in the end of 2001 I think and it remains the series that marked my interest more than all the others combined. It was the first series that seemed mature enough for an adult audience, and didn't have any moments where there was absurdity or straight out comedy in it. Some would hate this kind of anime as it isn't something to watch if you want to laugh your guts out or don't feel like thinking much. It's not as complicated as Serial Experiement Lain and far from it, but it still demands more than any basic anime that you watched as a child.
What I appreciate of the series is that it's complete and can't really be continued, as often are popular anime of today. The authors left a few unanswered questions in the storylines, but all in all, the mystery that turned around those two character enthralled me and made me want to know what will happen next.
This series even inspired my username here on AO back in 2002 and I still use it: Soldat of life, from "Soldats" and on my MSN "True Noir", this one easy to understand for those who watched the series. I even have an RPG character on another forum based from the character "Kirika" using a few tweaks from the ending to make her a potent character to develop upon as she has a LOT of holes in her past I can play with :P
Final words, I loved it, those who think there is any form of sexual development in this is false, it is dead serious till the end and well, I watched it 3-4 times in whole and might be watching it again in a month or two xD