Personally, I'm a huge fan of all the more "extreme" or "taboo" things found in hentai, as I see hentai as the ultimate medium for fetishes and honest expression. It's pure creativity, passion, and it's theoretically boundless. There's something spectacular about that simple fact, something liberating. Loli is one of my favorites, incest is okay (it can be fun, but it's more neutral for me), scat is almost like incest for me (though a couple of my all-time favorite hentai feature it prominently), gore can be interesting if there's actual story behind it, golden showers are fun, etc.