The best ROI it's hand down on online courses. Also memberships but less in terms of ROI, and more concerning having a good Free Cash Flow.

Why Online Courses? Becuase if you create amazing products that can really help people solve their problems and create value, you can price them at a premium price, and if you market them correctly, people will keep buying them.

I strongly suggest you read the book "Ditch Your Average Job Start a Business and Score the Life You Want" from Daniel DiPiazza