Points to keep in mind are:
1) First check the processor of the phone.the best processors are snapdragon processors.
2) Then check that the phone’s battery capacity.nowadays I suggest you to buy smartphones with good battery power i.e. about 2500 mah.
3) Check the android version.
4) most important if you are buying a 4G phone then please see that the phone supports VoLte feature or not.
Hope these tips work for you.thanku friends