I am not sure about what you mean with «based on Ubuntu». A desktop environment either runs on Ubuntu or it doesn't. Also, «the best desktop environment» is clearly a matter of opinion, therefore I will focus only on your bullet points.
- Touch-friendly buttons
- Cursor disappears when using touch input but shows where it was pressed
Ubuntu already ships with such a desktop environment, in fact it is the default one: Unity. It was designed especially for working with both traditional and touch screen devices. As a matter of fact, the first Ubuntu tablets being sold currently make use of Unity for their interface.
- Screen rotates based on accelerator
This is slightly unrelated to the DE. Screen rotation is related to the graphical server. Scripts automating this exist for different models (e.g. see this link for Microsoft Surface) but they are not universal. Your device might or might no be supported out of the box.
- Keyboard and touch pad automatically disabled based on how far back screen is folded
This would also depend on the level of hardware support for your specific device.