Which Medicare supplement to purchase?

I chose United Healthcare through AARP, plan "F". I have had this plan for almost 10 years and never once had a problem with acceptance or pay. I have used it on the road a few times and was never questioned. You do have to remember that the supplemental plans ONLY WORK WHEN MEDICARE ACCEPTS THE CHARGE. In my case, I have Medicare pass on the rest of the bill to the supplemental plan and the only issues arise when the MD or Hosp. tries to bill the supplemental directly. It is quickly fixed. So far, I have not had to pay out of pocket for anything I have had done, and I have had several hospitalizations and surgeries.
I am happy with my plan.