The manga has a bit of a weird story arc deviation halfway through (it doesn't change anything-- it's just bizarre). The rest of the manga, though, is so good, it physically hurts me. It's been finished for a long time. You can find scanlations online.
Well by Sales and overall popularity One Piece takes the cake and runs with it, trumping nearly double the sales of Dragonball in it's entire 34 year lifespan. Now that's impressive.
Read it and it is a spectacular manga, gets a lot of inspiration from Akira but does it infinitely better with a very good narrative. I just love when Oda packs so much going on into one page and how the panels just flow.
Attack on Titan, especially from Chapter 90 onwards when
the story did a complete 180 and delved into the nation of Marley and its inhabitants, and making us feel for supposed villains like Reiner, Bertholt, and the rest of the Warriors. There are no heroes in this story anymore.
Slam Dunk was a great manga too. The final arc was one of the best I've ever read :'(