No arc yet can top Thriller Bark for me. Not even Impel Down, which itself was pretty great. (RIP Mr. 2 Bon Clay)
TB was basically the perfect Scooby-Doo/Rocky-Horror/Young Frankenstein mashup ever. The scaredy-cats survive the zombie apocalypse to have dinner with Hogback and Cindry, Nami gets groped by no end of clammy zombie hands, the big-bounty Straw Hats make Cerberus their bitch (they don't do enough with Robin palling around with the Monster Trio IMO), Hogback and Moria compete to see who has the stupidest laugh onomotopaeia ever, Sanji's spontaneous combustion of love and secret Devil Fruit confessions, Armor Luffy, Nightmare Luffy, Tactics Fifteen, the eight-way Straw Hat gangbang on Oars, and the BEST USOPP FIGHT EVER. (Usoop vs. Luffy being a close second.)
And on top of all of that, there was Brook, who just about perfectly straddled that imperceptible line between annoying and infectious. (His flashback was also blissfully shorter and less cluttered/overblown than Robin's, and actually somewhat touching.) I rather like the anime version, to boot; Brook's voice is genius and the Binks' Sake insert song is really good, a very authentic-sounding pirating song. Bottom line, pretty much everybody gets to do something cool in TB (even Nami, when she's not getting felt up by zombies).
That said, now it's time for the Hatorade.
By contrast, I got honestly sick of the CP9 saga by the end. Way too cluttered and overblown in many parts, and most of the fights really didn't interest me all that much. I liked Sanji vs. Wanze and Usopp vs. Luffy well enough, but I didn't really care for most of the rest. Even the gimmickier ones like Zoro vs. Kaku and Nami vs. Kalifa, which normally are more my beat, really failed to interest me all that much.
The main arc of the saga was the main thing that was obnoxious to me. I am a big fan of cold, calculating Robin, who goes around behind the scenes/in the background in arcs like Skypeia and TB breaking people's necks, or deadpanning whenever Luffy and his friends do something profoundly stupid. I'm much less a fan of overblown emo Robin, who spends chapter after chapter doing the woe-is-me schpiel while being held by a villain who's (and I know this is a stretch for One Piece) actually too hyperbolic to be entertaining.
Robin's emo turn didn't even seem all that believable to me even considering her background; sure she's in the snares of the ridiculously powerful CIA (sorry, CP9), but at the same time, she's a very calculating sort about who she allies with, and up to this point she seriously considers the real prospects for victory of all parties involved. Surely by this point she had plenty of time to see that Luffy's quite strong enough to give Lucci a run for his money (this is a man who kicked the shit out of her old boss with only the help of his half-dozen goofy friends, and then for an encore he went to the sky to beat the shit out of God!)? She always had a surprising amount of charity towards Luffy's berserker "ADVENTURE GO FORWARD PUNCH BAD MAN GET ONE PIECE" strategy (more like an algorithm, really), and then all of a sudden she just inexplicably loses faith in him all because guys in masks and a narcoleptic jerk on a bicycle say foreboding things to her. I mean, really, who would YOU put your money on, a tiny little operation run by a kid who can blow through giant animals and self-proclaimed gods like wet paper, or the incredible might and resources of CP9 as headed by a twit like Spandam?
The whole "OH MY GOD CP9 AS THE SHADOWY ARM OF THE WORLD GOVERNMENT IS SO BIG AND EEEEEEEVIL" thing really got on my nerves. Even as a 9-year-old Japanese kid reading this shit, do we really need to hammer this out so hard? I thought Usopp leaving the crew over the Merry issue was a lot more compelling, but by the end of the arc that basically got lost in the shuffle.
The most obnoxious part of CP9 was the Robin flashback, though. Oh my God, I thought to myself as I read it, Oda worked in damn near _every conceivable[/] overused tearjerker cliche in the book – you've got the Cinderella/Harry Potter/Elephant Man "wah I'm a freaky little child unloved by the ludicrously bigoted villagers" angle, the kindly foster-father Dumbledore figure to kill off for canned pathos (they shoot him, THEN burn him to death with Olvia; that's extra points right there!), the absentee Bambi-mother who tries to be hardassed but betrays her maternal tenderness (and then they shoot her and burn HER to death, because Dumbledore alone wasn't cutting it!), the wacky giant friend with a ludicrous laugh ("DERESHISHI" more ridiculous than "FOSFOSFOS"? To be honest, Saul, I think you won it cold!) who gets turned into a popsicle after playing the "save my baby" card with Olvia, the tacked-on Snow White huntsman angle with Aokiji, and then finally a reprise of the "nobody loves me because I'm a freak AND a wanted criminal despite being 8 years old and adorable" angle. JUST IN CASE YOU FORGOT, YOU HORRIBLE, UNCARING READER.
I'm sorry, maybe if they'd just left the details of the Ohara Buster Call unillustrated (just leave it at "Everybody I loved as well as the people I didn't burned to death in the Buster Call" and focused more on her life as a fugitive, it might actually have tugged at my heartstrings. But the whole "let's work in every CONCEIVABLE tragic origin" spectacle of Robin's flashback was so amazingly stupid that I couldn't take it seriously at all. A rare misstep on Oda's part._