The one you can scale.I know successful business owners in the fitness industry, tanning salons, real estate, window coverings, maintenance, billboard, ATMs, etc.Generally, I would be skeptical of jumping into a business based on YouTube videos hosted by and featuring people that are likely primarily trying to sell you their respective courses.Small businesses can indeed be great investments, but they are hard.  

The main issue with rental basement is water if it has water issue usually it is a simple fix .Clean gutters ,discharge gutter  water away 10" away from foundation, make sure you foundation soil is slopped away from it .If you need to buy soil get a clay mix .Cut back any tree too close to the foundation look at the drip line that's how far the limbs are out from the tree .If all that is done still has water buy a sump pump  carry the water to day light away from foundation 10" to 15" .

Even with an all cash purchase, one would be lucky to get a meager 5% on your cash.

Sounds like you are on a the path so far.

Buy, Rehab, Rent, Refinance, Repeat method to pull equity from their first property and use it as a down payment on the next.

I hear the same thing at local meetups and on this site.  

@Shan Vincent I've used zumper.com for years without any issues, but don't 100% rely on anyone but me.

Read books like Set for Life by @Scott Trench and Start with Strategy by @Dave Meyer to get the mindset of starting on the table before going too deep into an asset class or something like flipping.

The timeline on that appeared to be potentially lengthy and costly. 

It shall be delivered full at the end of your lease or the amount to fill will be deducted from your security deposit.Something like that.Oil is tough on tenants because it costs a lot at a time to fill.