Graphics processing unit. The GPU is responsible for all the graphics intensive stuff on your smartphone including video rendering, playing games etc.
Random Access Memory(Volatile memory) in the smartphone. The amount of RAM in a phone is responsible for how many apps will stay in memory and will not have to reload.
Internal storage capacity
non-removable hardware storage inside the smartphone. The internal storage on a smartphone is responsible both for storing files, as well as the applications on it. The whole system runs on that internal storage which makes its quality important. Modern internal storages range from Emma 5.0, Emma 5.1, UFS 2.0, UFS 2.1 etc.
External storage available
Availability of a micro-sd card slot.
External storage capacity
Amount of removable storage that can be added to the phone to increase the offline storage. The external storage let's you upgrade the space on ones phone allowing them to store more and more files.
Availability of a fingerprint scanner as a biometric unlock. The fingerprint scanners have become the most popular and secure way to unlock smartphones today and will be used till other biometric locks become as secure and as convenient.