Hunter x Hunter is my favourite manga because of one reason.
That reason is that the two main characters grow during the entire manga.
I love Gon and Killua (could care less about the whining Kurapica or the annoying Leorio), and these two characters grow exceptionally much during the entire manga that you won't even recognize them anymore if you compare the first volume with the ant arc.
Gon is your typical goody two shoes that is naive, pretty dumb, but very strong and has a lot of talent (Goku/Naruto anyone?). Killua is the opposite; he is a cold blooded assassin (could care less if he killed someone), very manipulative and very smart (and also very talented).
Now these main characters look very stereotypical, and they're nothing special, because these type of characters can be found in any other manga. However, HxH breaks away from the stereotyping and develops the characters into 3 dimensional people. A lot of people hate the Greed Island arc (they go into a virtual game where they can really die and collect cards), but for me it's the best. Not only do I love the whole concept of the game (well thought out Togashi!), it's also where Gon and Killua changed drastically from their original personalities.
In the Greed Island arc we see how Gon changes from a naive, dumb kid, to someone who HAS to prove himself and has to use his strength to get there AT ALL COSTS (aka; he's a maniac), and he can't do it without Killua either. Killua transformed from a cold, manipulative kid, to a best friend that would do ANYTHING for Gon, literally anything. Gon and Killua have grown on each other so much that it changed them for good; the best friends would die for one another and it has changed their personalities.
No longer do we have the goody two shoes and naive child Goku .. cough I mean Gon who believes he can be friends with anyone. Now we have a Gon who would kill anyone without a second thought if they would try and hurt his existing friends (especially Killua). He loses his mind when he is in a state like that. It's such a refreshing turn on a stereotypical character like that. In the latest chapters (around 290) you can really see how scary Gon has become; you won't recognize the nice, naive kid anymore.
Killua's wall has broken down and went from a manipulative, could-care-less-about-the-world kid to a heartwarming person who would help out other people, and he will always stand next to Gon and help him out. He doesn't hide the fact that Gon means everything to him, and will always try and protect Gon (I swear these two are gay sometimes). Usually characters like this would never change so drastically like Killua did, which is why it's such a joy to read about both Killua and Gon.
So because these two characters grow so much during the entire manga, HxH has become my favourite because of Killua and Gon's friendship.
HxH can have the best, most thought-out battles in shounen manga, but for me, it's these two characters that count the most. Interesting battles won't make me keep reading, developing characters will. So if you are also interested how characters can change, this is definitely a manga for you.