For supplemental, I recommend a Plan F from one of the insurers ....it pretty much covers everything for doctors and hospitals, that Medicare doesn't pay for (which is 80% and this covers the other 20%, so your out of pocket is 0). AARP recommends United Healthcare, I use Blue Shield, but it depends on which state you live in. You can also get a Medicare Advantage Plan, but it works more like an HMO and I like to choose my own doctors. Always use doctors that are Medicare participants. When I first started on Medicare, I tried to collect for a non-participating doctor.....forget it, they never approve it (I subsequently found out from my State's Insurance Commission, that I was wasting my time), therefore I always ask the doctor's office.
As far as Part D (prescription), I use Humana, which I found to have cheaper premiums than United Healthcare and have been happy with them. I don't have alot of prescriptions and have found them good for my needs. They participate with most of the national chains and have their own mail order.
Medicare makes all this stuff confusing and difficult to understand until you get the hang of it.