This can be answered in many ways.
But your best bet is to write what comes natural to you.
If you follow your gut feelings you won't be sorry.
Ask yourself this question, (What am I passionate about?) What drives me, what do I enjoy writing about?
If it holds your attention and you're good at it, then it will probably hold the attention of someone else too.
Writing can't be forced or it will come across as such. Just stay true to yourself, and most of all have fun with it.
If it isn't fun or enjoyable then don't bother.
But to give you another answer, my answer would be screenplays. I love to write them, they definitely challenge me.