Man this sorta changes depending on what I've read recently. Gintama has a special place in my heart, despite me wanting to hurt Sorachi so badly when I'm translating it. Gintama manages to do more genres in one arc well than some manga manage to do during an entire run. I love that Gintama isn't about 'success' it's about 'getting by'. Gintoki and the other characters aren't super powerful, super lucky, or super relevant. Which is kind sorta how life is. Everyone's just doing their best to get by and life isn't always elegant moments traced with dreamlike ecstasy like most shounen manga would have fans believe. Life's tough, unfair, and sometimes all too short, and I appreciate Sorachi for not being afraid to be brutally honest. Plus it's a bit refreshing to have a protagonist in a Shounen who's my age. :-P
I echo One piece. I may be overreaching, but I believe that Oda has crafted an epic comparable to the Odyssey or the Iliad.
Hunter X Hunter is also amazing. A perfect antithesis to Shounen manga, even if Togashi didn't mean it to be one. Every time you figure Togashi is going to embrace a trope, he ends up perverting it into something completely unexpected.
I'd like to also say I like Fukuchi Tsubasa's works for being just....fun. Sure they aren't thought provoking, or even very original, but in these times where a Shounen manga seems to be judged to be good or bad on merits of how emo it can be, I really appreciate Fukuchi for just being himself, weird and offbeat, but always just tons of fun. Ueki, Takkoku, and Anagle are all the most fun I've had reading/translating manga, and are just great for kicking back an indulging one's inner child.