Least preferred way in the disease category would be any disease that slowly erodes the mind and body. Being unable to move or know where I am, who are those around me sounds terrifying, not to mention the burden (financial and emotional) it would be on my family. No thanks.

Now for "mechanical" I'm not totally sure what this means but a similar logic applies. Of the classic methods of execution I think getting burned alive would be probably the last choice, but I'd take that before slow torture that's for sure