The 2011 Hunter x Hunter is underrated and I might add amazing. I find that this is a shame, so many people brush it off as just another shounen, yet the anime has so much more to offer. Seriously, the quality of animation, the music and of course the story is great, and they fit the series perfectly.
The Heaven's Arena Arc, starting in episode 27 and going to 36. This is the shows the first departure from mainstream shounen. Here you have a drastic battle system change that is a bit of a breath of fresh air. It doesn't go full seinen until the Yorkshire Arc starting at episode 37 and continuing until episode 58.
Granted, I do feel the first two arcs do have some merit to them, even if they are a bit lighthearted. The current arc is a really impressive feat of storytelling.
The only thing that bugs me about it is that I feel like a lot of what could be more focus on dark themes are underscored until halfway through, around that point it starts to get a lot darker. This might have been an attempt to make the show more kid-friendly at first, I'm not sure, nor do I have a link to back it up.
While it isn't my favorite it is a cool show and I can fully understand why someone would call it their favorite show.