one is from 1999 and ran until 2001. It was created by Nippon animation. It only ran to the middle of the yorknew arc so it left on a stupid cliffhanger :lol. But nippon released some ovas to finish up the arc. Then they took a break to release ovas of the next arc called Greed Island, they completed greed island and have stopped animating HXH since then.
The other was created by madhouse and ran from 2011-2014. It's much better than the old ones and nearly flawless. I assume they still have the license because they still make art for the series (in mobile games). 2011 ran all the arcs it hasn't covered the most recent arc.