Arcs are like different stories within a single manga or anime. Like how DBZ has the Raditz Saga, Frieza Saga, Cell Saga, Buu Saga, Bebe saga, etc.
Cell Saga was definitely the best in DBZ. I loved the Androids. I don't recall them killing anyone. They just sort of went around messing stuff up. And Cell being one of Gero's creations was also a good drawback to the Dragon Ball days when Goku would constantly fight the Red Ribbon Army. One thing I really loved, there was this "oh, shit" moment in the Cell Saga where Piccolo says something like that "Gohan isn't like you. He isn't a fighter, he doesn't wanna be stronger. Right now, he's probably out there thinking why his own father is making him do this alone." And Goku goes from a totally confident expression to completely shocked or sort of regretful.
Bleach's best arc was definitely the espada arc too. (I haven't finished it, I'm still watching it on Toonami but I'm enjoying it.) Not just with the fighting but I loved the story and how it explains Aizen's experiments and how the Vizards came to be and their pasts with the Soul Society. everything tied in so nicely with that.
Naruto, I haven't seen the new ones but I remember a few years ago, I loved the arc where the Genins (Naruto, Choji, Shikamaru, Neji, Kiba, Gaara, and I think Rock Lee) are fighting Orochimaru's henchmen. I thought it really fleshed out their abilities and showed us what they were really capable of. Though I can't recall every arc (I'm rewatching it on Toonami but I'm still on the Haku/Zabuza arc) so this is subject to change. Though the arc with Orochimaru fighting the Third Hokage was pretty amazing too.
One Piece, this one is so hard. There are so many great story arcs, it's hard to choose. Not just in terms of fighting and story but emotion as well. The Whitebeard War had a great combination of general badassery and emotional depth. You get to see all the Marine Fleet Admiral, Admirals, Vice Admirals, Coby, all the Pirate Warlords, plus a bunch of old villains like Crocodile and Number 1, plus Whitebeard's entire fleet and all of his awesome crew members. Then Blackbeard shows up later, it was just amazing man.
Fairy Tail, as many unique and brilliant things this anime could have theoretically, none of it is really executed to it's full potential. But one arc that sticks out in my head is the edolas arc. The characters are transported into an alternate version of their world where humans don't use magic. And they meet alternate versions of themselves and there's this King who wants to steal the magic from their original dimension to fuel their world of dying magic. It was really interesting. And they also introduced some interesting characters like Sugar Boy, Hughes, and Panther Lily. And there's also sort of a feud with the Royals and the race of exceeds (flying cat things).