I've found it's best/least expensive to have a complete Medicare plan with part D included. The medicare.gov website has a calculator where you can input your expected visits, meds, expected procedures, etc. If you call you should be able to get a free booklet with the premiums for your area. I somehow got one automatically.
For my husband, who is healthy and uses nothing, the local (Highmark) BC/BS High Deductible plan - with no monthly premium - is the way to go. For me, and I need lots of stuff - spine injections, MRIs, CTs, blood testing, lots of meds, the same plan is best. I used up my deductible early in the year and didn't pay anything for the 10 days I spent in hospital for diverticulitis, or the knee and hand surgeries, or the phys. therapy. That's way better than paying 20% out of pocket!
Since docs are dropping regular Medicare right and left, if I want to continue seeing my docs I have to be on a supplementary plan.
Plans are like banks and grocery stores. You can't just keep the same old one without reviewing the prices every year.