What exactly are you looking for in the phone.. what features? And in what range you want to buy? That will help the people to narrow down choices and suggest suitable phones.
Smartphone with touchscreen would be the choice. For senior citizens, learning curve will be there in any phone that you buy, because you will have to teach them ultimately. But, touchscreen phones will be relatively easy to operate than button ones. It might take a bit of time, but touchscreen phones are good for senior citizens.
Speaking this from personal experience. Bought a Nokia phone with buttons for my mother, but the buttons are so hard, it's difficult to write on an app like WhatsApp. She can operate a touchscreen tablet much easily.