Medibang Paint Pro (my personal choice)
It's kinda like Paint Tool SAI, but free
Pros:
- Lots of good brushes both built-in and on the cloud
- Auto-saves by default
- Supports most file formats
- Cloud saving
Cons:
- Inconsistent compatibility with mouse-drawing
- Some premade brushes are locked behind a paywall

FireAlpaca
Also similar to Paint Tool SAI
Pros:
- Though it doesn't have as many built-in brushes, it's compatible with brushes made for MediBang
- Has an animation-making tool
- Supports most file formats
- Can be installed without administrator privileges
Cons:
- Inconsistent compatibility with mouse-drawing

I've only included ones I have personal experience using, as I can't really speak for ones I have not used. Feel free to suggest more if you know of any, make sure to include pros and cons.