- Namek Saga
- Soul Society Arc
- Marineford
- Pain Invasion Arc
- Dark Tournament Arc
Namek tops it for me. Call it nostalgia, call it genuine simple enjoyment. There's enough constant action or interesting character introduction to keep my attention. Then you get to the maneuvering with Vegeta in the first half of Namek only to hit the wall of fights that is Frieza in the back half. Giving us Super Saiyan, forever changing the direction of the series, really hits it home for me.
Soul Society's a fun arc with a good deal of drama, emotional baggage, cool fights, standard but epic shonen moments. The twist at the end is almost predictable now but it's still solid. The constant running into of new, strong characters to fight, the unique powers first explored on a wide scale, back then Bankai and meeting Captains meant something as well. Bleach's best arc. I only think it falls flat of Namek because IMO the finale of the arc is a little half assed. Aizen's appearance and departure was very abrupt and shocking but I do think it makes the pacing long.
Marineford goes over Pain Invasion Arc because Marineford has solid reveals, quick battles, an array of new character interactions and our first large scale battle. But also because it has consequences. The OP world is forever different because of Marineford, the protagonist theoretically was shaped by Ace's death, Blackbeard rose to power and Sakazuki took over the navy. Shit happened because of Marineford's outcome.
The pain Invasion arc too neatly brushed everything under the table with Rinne Rebirth. What little isn't—Danzo, Tsunade in a coma—is resolved by the end of the very next arc, so by the time the final saga begins, the fact Konoha is recovering from a large scale assault is not relevant to the story.