What exercise machine is best for diabetic patients trying to get fit?

Anything! It is important for diabetic patients to exercise. Any exercise! pick something you enjoy doing and exercise frequently. Cardio exercise (walk, run, bike, etc) tends to help more with weight loss more than weight bearing exercises.
Any. You are in this for the long haul so choose any machine that you think you can use for the rest of your life, that is, anything that you can tolerate. In general, machine that keeps pressure off your joints are better tolerated: elliptical, water treadmill...