That is certainly a matter of opinion and the model which would be "best" for you would not necessarily be the "best" for someone else. The most recent models would be the XR($750), XS($1000) and XS Max($1100).
I am sure there are people who would say each one of the above devices is the "best" Apple phone.
I, too, am a grandma and just replaced my Iphone 6s Plus with the XR. It is a little bit smaller, yet the screen viewing is larger. No more bezel and no home button. The face recognition is perfect. No longer do i have to try to get my finger print just right or enter a passcode. I just got an excellent deal on mine, plus I was able to trade in my old phone for even more savings.
Getting a new phone involves careful research, Grandma_Lou. With the rising costs of phones these days, it’s even more important to get one that meets your needs. You came to a great place for some advice, such as that offered by rschnoor and rempson. Here’s a link to all our current iPhone models: http://spr.ly/6608Ed8Aa What is it about your phone that you really love? Is there anything about the current phone you wouldn’t want in a new phone?