Thriller Bark takes it for me and i think i'm a bit biased because it's my favorite arc. However i do belive Thriller Bark was something special in it's ending because it's not a cut and dry ending like most others. Example.
Alabasta- had a great ending but it was expected (except vivi not joining, that was always 50/50) and common. Luffy beat croc, zoro beat the second strongest and all the mains are exhausted but alive at the end. It was done very well no doubt, but very common.
Skypedia- Very poetic and memorable but once again very common, luffy has alreayd fought enel once at this point (and whooped his ass) so we know he can beat him, it's just a matter of getting there. The fight is ok and all the loose ends being tied up worked very nicely, but at the end of the day it was just a trial the straw hats passed with alot of help from the skypedians.
But THRILLER BARK-
First, the crew didnt even want to be there, which automatically makes it different from every arc before it because at no point in the arc do they have any goal on that island other than survival and getting the fuck out. People often tell me that one piece always follows the same formula and that is boring but this arc proves them wrong. At the end of the arc, after taking out the henchmen in matchups we did NOT see coming, the crew is faced with the behemoth Odz and after each and every one is pushed to their absolute limit they topple him with the most epic teamwork i've seen in a shonen. Only to find that the actual arc BOSS is up and ready to go again, more powerful then before and ready to destroy them all and they only have a few precious moments before half of them are instantly killed. Luffy pushes himself past his limit to stall Moria long enough for the crew to live and even then they are still going to die but their will is strong enough to literally pull themselves together. And just when you thought it was over Kuma comes and wrecks their shit and gives Zoro one of the most badass moments in the entire series and through this all Luffy is passed out, imagine that the final battle of the arc and the Hero is unconcious and needs his crew to save him. One of the biggest mysteries in the series is answered and then immediately another one is raised. (Outcome of Ace vs. BB / Is Kuma good or Evil) And we still get the customary party sendoff and the arc manages to STILL end on a chilling note with the creatures in the fog staring ominously. At the end of this arc the crew is alive but Zoro is possibly perminetly injured and we dont really get the sense they won and we dont even know why they didnt lose. This makes a better ending to me than anything customary, no matter how epic oda may write it.