Before I go into mine, I'm just gonna clarify that I don't hate any of the arcs in this series; I've only been into One Piece for like four months, but it's just so incredibly good (in my eyes) that I don't find a single one of the arcs to be truly bad. For me, my "least favorite" arcs translates to "the ones I found most forgettable and/or cared about the least". With that being said:
Least Favorites (in chronological order):
1. Long Ring Long Land (a.k.a. Davy Back Fight): Mainly because, other than Aokiji at the end, it wasn't relevant to the plot at all. I didn't mind it, but you knew Luffy wasn't gonna lose a crewmate, so that robbed it of tension. Also hated seeing him get beaten up that badly by Foxy. I wish we could switch it with the anime G-8 filler (and add stuff to tie it into the plot), so that G-8 was canon and the pre-Aokiji portion of this arc was actual filler.
2. Impel Down. That might be controversial since that's many people's favorite, but for me, the crew and their relationships and shenanigans together is what I love about the series, and I missed them for the whole thing. I think with Amazon Lily, the arc was short enough and the separation was fresh enough to not bother me as much, while with Marineford, there were so many "Holy shit" moments and plot twists that I was able to be distracted from missing the crew. I also wasn't big on all those chapters of running around trying to get to Ace, only to fail in the end when he was taken away.
3. Fishman Island: I actually didn't have a problem with the Straw Hats completely owning their opponents since it was important to show how much they had improved during the timeskip, but even still, the fights really weren't very exciting, and I didn't feel any tension. If anything, the scene of the royal family and Jimbei captured (or pretend-captured in the latter's case) was dragged on for way too many chapters; I was like "get on with it, we know the Straw Hats are gonna show up and save the day, so hurry it up!" Didn't help that, while Arlong was an enjoyable, "love to hate" type of villain, Hody was just a hate sink and not even remotely enjoyable as the big bad.
Funny enough, the first time through, I would have put Skypeia on the list (which I know most people here love) because of how forgettable the characters were. I really did not give a shit about the Shandorians' problems for most of the arc because I found them unsympathetic until the end, and the Skypeians were just boring. The anime for the arc was actually really good; it made me enjoy the "adventure" aspect of it, and I now like and appreciate it much more.
Favorites (in chronological order):
1. Early East Blue arcs (namely Romance Dawn, Buggy, Baratie): I didn't include the Kuro arc there because it was my least favorite of the East Blue saga (though I still liked it), but I actually still go back and re-read the others sometimes. Even though One Piece was still coming into its own at the time and didn't perfect itself until Arlong Park, I loved being introduced to the original Straw Hats and seeing how Luffy inspired them to join him. Even back then, his character was still pretty consistent with how he's depicted throughout the series, which I appreciate.
2. Arlong Park: Pretty much what the original poster said. As I said above, I liked the entire East Blue saga, but Arlong Park was the arc that confirmed for me beyond all doubt, "yup, this was the right series for me; I made the right choice". I, too, don't have anything at all about it that I dislike; great pacing, great fights, great emotional moments, great backstory for Nami.
3. Sabaody Archipelago: Even though it ended with the Straw Hats being split up (and as I said, I missed them throughout the rest of the saga), I liked the arc itself a lot. This arc had all the current crew members together, and while Kuma splitting them up was definitely tear-jerking, it was emotionally investing and had me at the edge of my seat. This arc introduced lots of new characters who (especially the Supernovas) were very interesting and are plot-relevant (unlike Skypeia), and there weren't too many of them (unlike Dressrosa). Also had plenty of good setup and forshadowing, and was a good length with the right pacing.
Unlike quite a few people here, I actually liked Dressrosa, although I'm guessing I benefited hugely from getting to read the whole thing at once rather than have to wait, week by week, for it to unfold. I found Law to be pretty intriguing, so learning about his backstory and motives was enjoyable, as was his antagonism with Doflamingo. On that note, I also found Doffy to be a very enjoyable and genuine threatening villain, unlike the previous post-timeskip big bads. The main things that keep it from making my favorites list are that 1) even after getting to read it all at once, I still thought it was too long and dragged too much at times (I don't even want to begin imagining how the anime must be, since I know they've been padding the shit out of it); 2) As I said above, their were way too many new characters to be able to care about or even properly keep track of them all; and 3) those new characters came at the expense of half of the Straw Hats; I wish they'd been around to fight some of the Donquixote Family officers, instead of so many newbies. Still, I liked it, and it was definitely my favorite New World arc so far.
I'll have to come back later and see if Zou or other future arcs make me want to change any of these lists (hopefully only on the "favorites" end).