Scripting is my strongest attribute; I can pretty much script anything to do with flags, vars, battles, movement, setmaptile and all of that good stuff. Lately I've been inserting tiles and music to make my hack more graphically and audibly appealing. In my hack I have scripted up to the third gym or so and doing more as we speak. I should get round to posting a thread in the Progressing Hacks place but always find myself sidetracked with the final touches, the final final touches and the final final final touches, if that makes sense.
I'd have to say ROM Hacking. I've been lurking the whole scenario of it for over a year now. I've always been fond of the Pokémon game series since R/B/Y and seeing the opportunity to make one of my own was too awesome to resist.