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.
According to me: Here are some of the most profitable blog niches: Personal finance Health and wellness Beauty and fashion Food and cooking Travel Technology Remember, these niches are just a starting point, and other profitable niches may align with your interests and expertise. It's important to choose a niche you are passionate about and knowledgeable about, as this will make creating engaging content and building a successful blog easier.