Mine is porn addiction. I don't ever want to become a coomer but I think I've became that already a few times in my life. I shamefully have watched porn, saved porn images and visualized people who're probably not as into porn as I was. I really do wish to be done with porn, it's done nothing for me. I've masturbated for many years and I feel like it has hollowed out my mind. I don't even get that much enjoyment from masturbating as much and the porn hasn't really gotten any better so I guess I can say that I've seen porn when it was at its best when I was younger and everything. Now all of it is just loli shit, artificial shit and that's gross or the fetishes have gotten too niche and unappealing. I look around me in porn communities and I haven't found anyone worthwhile to speak to or associate with. Everyone is six feet under in porn that there's no way for them out.

I don’t have any answer to the “hardest addiction” thing but OP, I don’t think watching a lot of porn is inherently a problem. Sometimes people just masturbate because they’re bored, in which case you should try to get some hobbies you find more fulfilling. But if you’re masturbating because you’re horny, why not, it does no harm.