First and for most thing in buying mobile phone is priority. You select the criterias for buying mobile phone.
Budget - This point get the first priority. If you have tight budget then you have limited option available.
Usage - If you have heavy users then select high power back up battery phone.
Type of usage - This is the another important thing is that if you are a professional user or gamer then you select a high performance phone with high RAM and internal memory.
Brand - Brand is to be consider because it gives a quality and performance.