For me the most profitable is the one where you can create good quality content and you enjoy writing about it.
profitable in what sense? if you're talking in a purely financial sense, the most profitable niches tend to be related to fintech or adtech, but as shabbir said the most profitable - and the ones that are going to appear higher up on a SERP - are blogs with quality content that's serves a specific audience interest.
SEO is a good niche but it's too crowded already and many competitors. Your website will need some serious marketing.
