I understand that choosing a microcontroller is all based on the needs and that there is no perfect Mcu.presently I have to select an MCU for a robot my team is building for the 2018 Abu robotics competition and I want to know how does the specs matter like what clocking speeds are ideal for what applications etc... while selecting one.
Typically a decision like this is made based on a list of design requirements. Have you listed these out? What do you expect the MCU to do? How many peripherals will you have? Since you have a deadline, I would focus first on getting something to work very quickly using a simple board such as an Arduino or teensy. This buys you two things: (1) You have something working, even if it doesn't have all the bells and whistles you wanted. (2) You learn your design requirements thoroughly enough to pick a good part.
Based on their 2017 rulebook, it looks like they don't say anything about how many processors you can have. In that case, I would recommend using something like an odroid for your higher level computer, which will give you enough power to do some lightweight computer vision and all of the planning/SLAM you could want. It has the added bonus of running linux, which gives you access to ROS rather than reimplementing everything in embedded C on an MCU.