Molina vs UnitedHealthcare vs Wellpoint: Any Medicare users have any advice or recommendations?

I am trying to help my father pick a healthcare plan who lives in Houston, Texas (I am out of state) but I don't find much information at all about Wellpoint and pretty bad things about the other two. I know insurance is a very divisive and turbulent topic so it's hard to know if the complaints are just about insurance in general or these specific plans.

I'd appreciate any advice or personal recommendations you can give me. Thank you in advance 🙂