The best, most rewarding and mostly intricately conceptualized manga/anime of them all, methinks — with a fictional universe that can be compared to Tolkien's legendarium or The Wheel of Time in terms of the scope (and scores higher than either on the uniqueness scale). Not for nothing is it now the
2nd highest selling comic of all time after only Superman. But you will have to be patient and forgiving with it at first go; unlike Shingeki no Kyoujin (which is set in a tiny little world and engages your senses with the very first episode), One Piece is deliberately meandering and takes a while to get off the ground as you're exploring a much grander verse with a million different building blocks, has a peculiar art-style and can be tough to take if you don't appreciate its innate sense of romance and absurdism.
Though when it does gather stream, it only gets better and better...and at practically every level too (be it characterization or world-building or story-telling); by far the greatest example of a long-running shōnen manga/anime maintaining a consistently excellent standard to go with endless mystery and intrigue and foreshadowing — instead of stagnating or falling by the wayside like Naruto, Bleach or Dragon Ball. I could honestly argue that right now (after 1019 chapters and 983 episodes over 2½ decades) is the best point in the entire series, and things will get exponentially better as there are so many adventures to be had and so much stuff that needs to be resolved.
Getting into it at this point will be quite challenging given the sheer ever-growing scale of the enterprise — no doubt about that; but if you decide to stick with it, this is a neat tool for tightening things up:
One Piece Filler List. BON VOYAGE!