I didn't see one of these threads, and thought it would make for good discussion.
For me, it's a tie between Thriller Bark and the Vongola Rings arc in Reborn!
I admit, I didn't like TB at first, but it's grown on me. The Odz fight is getting tiresome, but I know it'll go out with a bang. And then there's Bear and Moria to look forward to.
The Vongola Rings arc had that kick ass Shonen tournament feel, and did it to the extreme. Hibari one shotting a really hyped up baddy was awesome.
Soul Society (what got me, and most of the people who I know that like Bleach, into the series, and hooked us into it.)
Chimaera Ant in HxH, when I finally pushed myself to finish read HxH, the Chimaera Ant arc is freaking great, and I can see why it's so damn popular.
Love to hear everyone elses favorites.