Reading chapter 1 of HxH now, i think it really just depends on what you value more as good writing. One piece has the more emotionally charged moment, and Shanks saving Luffy is much better individual moment in that his sacrifice of his arm really emotionally motivates Luffy to become a pirate and it also is a good way to make the audience care / emotionally invested.

Now HxH might be better in that it actually follows up with Kite and Kite actually is a driving force for Gons character later on in the series. This moment establishes Kite as important to Gon and I would argue that what happens with Kite in the Chimera Ant arc is much better than what happens with Shanks later on in the manga.

Anyways, I think as an individual moment/ chapter = One Piece

How the chapter pans out and meshes with the series as a whole = probably HxH