Something you are good at and something you want your channel to be about. My first video was a turkey animation because I like animations. Don't make it too complicated.
also, make sure what you are doing is specific and not saturated, but is interested by a small group of people. For example, making a generic video on call of duty will barely get any views, but making a video on something cool in a less common game will earn you a few hundred views. take this example: when I was 13 I made a 2 minute minecraft video on a specific minecraft server. it has 140 views. 3 years later, I made a animation about thanksgiving and it got almost no views, because there are just so many animations mine is hid behind everyone else.