I know buying a smartphone is about spending a huge amount of savings. So we want it to meet our needs and should be best in everything. I personally bought some smartphones. I recommend to check these things before buying a smartphone.
Top priorities:
1. Your budget: never spend more than your budget to get a top branded smartphones like apple or samsung. You have many alternatives such as xiaomi, huewai, asus, moto, lenovo which will give you same specifications for best price.
2. Storages: nowadays 32GB ROM( internal storage) is recommended and 2GB+ RAM is decent for smoother android experience.
3. Display: it should be minimum 1280* 720p HD display it will not look grainy when you see it closely. If you want to use VR headset I recommend to prefer a 1920* 1080 Full HD display.
4. Battery: it should be 3000mAh or more because now everyone want to use phone more and more with lesser charging time.
5. Camera: if you want a excellent camera go for a dual rear camera phone with 13MP+ resolution. Front camera can be 5MP+
autofocus, HDR, depth mode make it more clear.
6. Sensors: every phone now has a accelerometer, proximity and light sensor. But a good phone must have compass, gyroscope, magnetometer and most important fingerprint sensor.
7. Connectivity: it should support all indian 2/3/4G Bands and should also has VoLTE.
Wifi dual band make it great
8. Other features: Some extra features can be Quick charge, wireless charging, gorrilla glass protection, water resistant etc
Hope this may help you choosing your phone.