One Piece! I'm dreadfully behind (too much time drawing, not enough time catching up on One Piece), but I love it so much.
I mainly watch the anime, because I love the way it all looks in motion and the soundtrack is great, and I've been watching it pretty much regularly for, uh, twelve years now? I just love it so much!
What I love most about it - and the thing that attracts me to most shounen series I enjoy - is the sheer, unbridled feeling it's packed with. One Piece is packed top to bottom with a kind of fierce humanity and passion that just gets to me. OP is full of people who cry their goddamn hearts out till snot comes out of their noses, who bleed and fail and get beaten down and get back up and who go to die for what they believe in, and what they believe in is a better world. It deals with big, serious themes like slavery, totalitarian regimes, the fragility of life, taking responsibility for the consequences of one's actions, the repeated cycles of violence breeding more violence, etc. - and also has a main character made of rubber, another who is a giant cyborg in speedoes, and a repeated gag of people sticking chopsticks in their nose.
There's something for everyone!
I have dipped my toes into a bunch of other shounen - Gurren Lagann, Yowamushi Pedal, Eyeshield 21, FMA, Bleach, etc., - but One Piece is my one true shounen-love.