In One Piece, Eiichiro Oda goes for the slow burn, with storylines building and building and playing out over volume after volume until they explode in an amazingly satisfying climax; it's a pretty amazing spectacle to witness. The Water Seven/Enies Lobby arc is the longest story yet, stretching over fourteen or so volumes, and while it did seem to drag at times (the obligatory one-on-one battles between members of the Straw Hats and CP9 seemed to slow things down for a volume or two), the big finish in volume 44 is something to see, bringing everything to a close in spectacular fashion, with the Straw Hats facing off against a massive threat while Luffy struggles against his toughest opponent ever, Rob Lucci. Oda throws in excellent character moments for pretty much everybody, but my favorite might be a scene in which Luffy seems to be beaten, but he gains the strength to keep fighting from Usopp, who drops his Sniper King persona in order to encourage his friend:
Being a shonen manga, the big fights usually come down to determination and strength, with the power of friendship often providing the motivation to win, but this is such a moving example, with Usopp overcoming his shame at having quit the crew in order to convince his friend to keep fighting. It's just one powerful moment among so many, but since it hits on the series' theme of friendship, it's the one that sticks with me the most. Wow.