No such thing as best niche. It's like asking what's the best car. And nobody is going to give you their niche. Just like nobody will give you the keys to their car.
All niches are good. Some are better than others. Experiment with a few, keep what works, throw away what doesnt. Look around on IG and see what other people are doing. If it looks like they're doing well, clone it. IM is all about failing and trying again.
Since you're on OG ads, don't they give you hundreds of niches to chose from with landing pages and everything set up for you? Pick a few that look good and start promoting them. Eventully you will find a winner. But the real big winners are usually the ones you make yourself, not clone/rip from someone else.