Favorite digital design software?

I have a quick question for anyone out there using digital drawing software to create designs.

I love hand sketching my pieces before fabrication and usually do it pen and paper but I was curious if anyone has recommendations for a digital sketching program for iPads? And are there any that can be incorporated with CAD/3D printing if I decide to produce the piece that route instead of 100% handmade?

Thanks artists, have a great weekend!