I'm slowly working my way through the Battle Royale series, I'm up to 12 so far, but I think it peaked at 9 and 10. It's still really good though.
The first graphic novel I read was Watchmen, so that's got a soft spot in my heart, but the Sin City series is really good too, and so is Maus. 300 by Frank Miller is too short, but it's a damned good read, and has great art, so I think it's one of the best I've read. The Essential Punisher is probably the one I've read the most times, though, as it's just insanely fun to read. V for Vendetta was really good the first read though, but by the 3rd time, it can be rather annoying. It's fun to pick up every couple of months and read in one sitting, which means it's above average, I suppose.
I'm not much of an authority on graphic novels though, I've only been reading them for a year or two, so I've not read that many yet.