Part 1 had a lot of creativity into it's world building such as creative environments and the history of the world. This still had flaws such as him offering cheap excuses to explain how the climates of islands are so drastically different, which leads into his flaw in never doing enough scientific research. Sure, such isn't essential for fantasy, but it does cause some suspension of disbelief when it comes to things such as Luffy not bleeding profusely from cuts in Gear 2nd (which would even make things more interesting by adding an additional flaw to the form). His characters always tend to have colourful personalities and a detailed past. However, Oda struggles to develop them in the current story beyond the arc where they're introduced, keeping character traits for so long they can break immersion (eg. Zoro getting lost, Luffy always splitting the crew up with each new island, etc.) or outright brushing over some important events as minor inconveniences; Luffy and Usopp's friendship returning to the status quo after Enies Lobby being the prime example. This shows that Oda tends to either care more about character popularity than development, considering typical fans don't like to see a character's personality change, or that he gives his editors too much influence over his work. I doubt the latter is that prevelant, considering that Togashi's direction of taking his characters wasn't altered by Jump and were allowed to be fully developed (bar Gon's regression after getting healed by Alluka), or how Araki still went ahead with killing major characters off until his work in WSJ ended.
Despite having a lot of creativity in it's setting and character goals, One Piece also has it's fair share of lacking creativity in certain elements of the story and characters. For instance, a fair bit of terminology is taken directly from things such as mythologies or fairy tales (eg. The Admirals' code names, the Ancient weapons, etc.), with many such ideas not being fleshed out enough to explain how they fit into One Piece's world. For instance, we see plenty of nuns, yet aren't given details on how similar or different they are to Catholic nuns in our world. Still, I can excuse this point as minor nitpicking, especially for a series with enough content to surpass a 20 year run without feeling milked, though not as much as many arcs having a repetitive formula. Most of the time Luffy will split up the crew and either get ambushed by an enemy trap (Alabasta, Thriller Bark) or mistaken as an enemy by the island's populace (CP9, Amazon Lily), which can make it quite infuriating that supposedly smart characters like Nami, Usopp or Robin never offer an alternative initial plan.
Part 2 I don't have much to say on. The flaws it's panned for were already there in Part 1, but just became far more apparent and vastly outweighed the good aspects, unlike part 1 which was for the most part an enjoyable series. The lack of character development became even more annoying after the events of Marineford should've ushered in changes for more than just Zoro, the names for new characters were taken from pre-existing works even more than before (The Kraken, Otohime, Loki) and the predictable formulas of arcs got up to the point where some seemed like pure copy-pastes of previous ones, only made far less engaging (Fishman Island = Arlong Park, Dressrosa = Alabasta).
I still consider Oda's writing quality decent for many similar reasons as Toriyama's better attributes, such as creative settings, unique items of the world and colourful characters. However, his ideas would need to be executed far better to be considered one of the best mangaka.
