BOTH! So very much! I really love a lot of her English songs, but Heart Station is one of the best albums ever, so...I vote for that.

i like both though

YOU ARE /EVIL/. I LOVE BOTH SO MUCH aitj;etiw
Usually, I prefer her Japanese work, but I picked her English work because my favorite Utada song EVER is Kremlin Dusk, which is English. If you want a quantity version of favorite THEN I'd probably pick Japanese because I much prefer the lyrics--and usually the music is perfect either way. I don't like a lot of her English songs though, because they normally try to dive after the "main" crowd instead of people like me who love her unique songs.