My favorite japanese band hands down is
maximum the hormone. i don't watch anime, and i liked this shit before deathnote was even released in america. i first heard their single 'koi no megaraba' in a japanese supermarket, and i've liked em ever since. they would be a horrible band to try and learn japanese from though. my japanese friends say they can't even understand what they're saying most of the time.
i dont really care for m-flo, the pillows are kinda nostalgic for me, and theyre much easier to understand. i used to hear them on radio stations. fuck gakt, fuck all male popstars. and let us all agree: FUCK YOU SMAP

. japanese rap is horrible. dont let anyone covince you otherwise. i remember watching japanese mtv and these fucking japs are the biggest poser tools. they fall way short. the only japanese rapper i will say that is half way decent is hannya.edit: and that's only because he's hilarious. i am going to say that most japanese music is just fucking retarded. i was pretty big into the osaka underground scene however. there are some talented artists. 2 of my friends are in different hardcore indie bands. watching their shows was a fucking blast.
they both sing in english so check out their sites:
EardrumSabnack