Also I just re read arcs and stuff all the time so I keep up and know what is going on all the time. xD
I have just started Billy Bat, and already at chapter 8 you start to say : You're damn Urasawa, what other crazy story are you writing !!
I have only have read Monster and Vinland Saga ... and though i found Monster a good work, the end was not my cup of tea. Too subjective.
So i am going to vote for Vinland Saga. A manga, that i already follow for quite some time, and that managed to continue being interesting even though it's monthly. It's really difficult for me to keep interested in a monthly manga ... but Vinland Saga is really good, and definitely a must read if you appreciate Seinen.
I voted Blade of the Immortal, one of my all time favorite manga. One of the few manga that can be placed in a literary genre (modernism) rather than just a manga genre.
I voted Vinland Saga, the greatest manga i've ever read, not more, nor less. Thorkell: best Mad character of ever. Askeladd: best calculator character of ever. Makoto Yukimura: a legend.