What is the best phone for seniors?

The answer to that question can depend on a multitude of criteria, none of which you specified, except for age. Usage, coverage, any physical disabilities, etc. There is so much to put into the criteria to determine the answer to that question. Your research is better made elsewhere too, since this is a user to user support forum for technical questions. Not trying to blow you off, but the responses here without additional input would be useless.

Older people are no more a uniform group with uniform capabilities and needs then are "middle aged people" or "young people". There are older people who have more technological skills and interest than some twenty somethings (many of those "older people" are in these forums, by the way). Then there are some older people who would really not have a cell phone at all but their children insist. A better question would be, "Given that a person has x level of interest in technology, needs a, b, and c features, would an iPhone be a good phone for them?"