The best long-form niches for turning views into subscribers are usually the ones where people feel they will need your channel again, not just watch one video and leave.
In general, the strongest ones are:
tutorials and software guides, educational/explainer content, finance/business, skill-building content like fitness, language learning, music or drawing, and deep niche commentary where people come back for your way of explaining things.
The reason these convert well is simple: the viewer is not only interested in one video, they are interested in solving the same kind of problem again in the future. That makes subscribing much more natural.
So the best answer is:
tutorial, education, and skill-based niches usually have the highest view-to-subscriber conversion for long-form videos.
Entertainment, random viral topics, and broad reaction content can get views, but they usually convert worse into subscribers because people often come for one video, not for the channel itself.
If you want the best long-form niche for subscriber conversion, pick something where viewers say:
“I’ll probably need this channel again.”