Hm, there is none in the Shounen genre. Not even Dragon Ball.
Some Seinen titles better than OP:
Monster
Fullmetal Alchemist
20th Century Boys
If we're talking HxH, I think the anime is much better than OP, but not the manga.
If we count anime exclusives and/or LN adaptation, I'd name:
Re:Zero
Steins;Gate
Cowboy Bebop
I didn't read/watch AoT, Death Note, and Berserk, so couldn't comment on them. But I heard that the ending to AoT and Death Note is either average or divisive, so I am not certain I will ever start following either of them.