First of one should be clear about your requirements from your cellphone. Some of the requirements are listed below: -
RAM minimum 1 GB. Maximum 2-3 GBs if you require your phone to do multitasking.
Processor should be atleast 2.2 ghz , so that there is no lag in multitasking.
Camera - you should go for atleast 8 mega pixels camera atleast.
Battery back- there should be atleast a backup of 3000-3500 mah.
Service quality. When you require a phone then you should take care of its repair and maintainance also.