I don't think "life hacks" is a top niche, not that it was 3 years ago either. It's a niche taken by the big dogs from a very long time ago. Same as "how to". And how is "how to" different from the "life hacks" category?
The mysteries of the Universe.
I'd say the top niche is the niche you're best at, and the niche that ain't oversaturated like a bad Instagram filter on a Saturday morning.