I've read shonens,
I've read seinens, ( including every Urasawa except Master Keaton )
I've read gekigas,
My favorite manga character is by far Doflamingo.
However saying he is the best is foolish since the notion can't be objective
About Monster, ironically, I think people reduce too much the breadth of the manga, reducing it to the mere Tenma vs Johan. I think Monster is more impressive considering all the hidden relationships within the 10 main characters ( let's say Leichwein, Tenma, Dieter, Johan, Eva, Robert, Runge, Grimmer, Anna, Poppe ) than only the relationship between Tenma and Johan/Johan's very own story. In that sense, the conclusion act is masterful.
I enjoyed Monster as much as greatest Monster fans, however I enjoyed less Johan than greatest Monster fans. I was more blown away by the web weaved by Urasawa.