Grossly simplifying and considering brands with all else being equal, Motorola has historically had great RF engineering, Nokia is good, LG and HTC would be goo

Grossly simplifying and considering brands with all else being equal, Motorola has historically had great RF engineering, Nokia is good, LG and HTC would be good to average, and Samsung and Apple would be average to mediocre. This is truly a gross simplification, as all else is not equal: it depends on the specific model of phone (sometimes differing greatly between carrier), the bands used in an area, etc. Ultimately, the only sure option is a "live fire exercise". Apart from that, maybe try a reddit subforum for where your father lives and see if they have good, local advice?