First of all Micro-Controllers are a specific type of processor that has ports allowing it to have programmed I/O. the rest of the list you offered are companies or architectures that have been used in microprocessing projects.

Pic for instance is a RISC micro-controller, because it's instruction set has been reduced in order to streamline production and speed.

PIC's tend to have small word sizes and be used for small control projects where tight control of input/output is important, but where the limits are not so severe as to preclude the use of a C language compiler.

Intel is a manufacturer of a wide range of chips from CISC (Complex instruction set) to RISC including DSP's etc. each processor family has it's own assembly language and word sizes tend to max out at 64 bits.

ARM is the name for a specific family of chips it has a reduced instruction set, but tends to have 32 or 64 bit word sizes.

You say it is a mega project but you want flexibility and speed, but really it is the nature of the data that is flowing through the processor that determines what is the most flexible and speedy choice.

Small snippets of data, in large parallel arrays require a different type of processor than large blobs of data in small arrays, and your flexibility can come from one processor that is faster than the data-throughput needed, or from a fleet of smaller processors that divide and conquer the problem.

precision of the response is an important consideration, if you need control at the bit level then you probably want something like a micro-controller but if you need control at the datafield level something a bit larger might be of use.

Is the data signals, or words in a text. For instance doing fast Fourier transforms is much easier in a DSP than in a micro-controller because the architecture and command language is optimized for the application.

Consider now the speed of the processor. Speed ranges from 1 MHZ to multiple

Gigahertz, but the faster speeds come with larger word sizes as well. A microcontroller will max out at about 16MHZ, a general micro-processor with a 64 bit word and 16Gigahertz operation is possible but only in the more expensive ranges.

It is sometimes necessary for flexibility to move away from a processor altogether and go to programmable logic, but if you do do not be surprised to find yourself mixing in a programmable logic device with a micro-controller.

A recent tear-down of an embedded system for a popular item showed a two board solution where one board was a custom chip, and the other board was a micro-processor of ancient (and therefore especially cheap) design with 1-4 mhz clock speeds. The microprocessor board was used to supply power control for two servo motors and so didn't need to be faster.

It is not clear from your question what you mean by a MEGA project, since there are multiple uses of MEGA in this general area. If you are referring to the Arduino Mega then your decision is already made; that system is based on the Atmel ATMEGA microcontroller, which is similar to the PIC but has a different internal architecture.

If you mean something different then please clarify. 

The previous answer provides useful background and information about CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer) and about many aspects of cyber-physical (embedded) computers in general. I would add one aspect to this--whether or not you are using an operating system. Microcontrollers such as the PIC, Atmel and Freescale units are typically programmed directly in C, they do not run an operating system. Larger systems, based on 32 or 64 bit chips from Intel or ARM almost always run some kind of operating system (often a variation on Linux). This has implications for the size and cost of the system, but also for the capabilities of the system. It is much easier to add networking, security, file handling, graphic interfaces and similar facilities if you are using an operating system. It is easier to directly control IO and do precise real-time control if you are not using an OS. Larger projects will typically combine both large and small systems. The small systems (typically microcontroller-based) will control sensors and actuators and deal with local real-time tasks, and the larger systems will communicate with the smaller ones and the outside world. 

First i would like to thank Mr.Graeme Smith and Mr.Richard G. Helps .

To clarify the doubts i want to say that i am working on two robots which have many functions to do ! Ex :- image processing, finding a path , etc. 

Mainly i was asking for a solution for a huge Robotic system

You probably want to go with a mixed system then, an operating system for non-real-time tasks and micro-controllers to handle the real-time sensors and control the servo's or stepping motors. You can run your image processing on the operating system, along with the pathfinding etc. and then use a small bus like I2C or SPI to talk to the micro-controllers. Depending on how complicated your non-real-time stuff is, you might want to consider a multi-processor or multi-core system, but this will increase the size of the robots to fit the hardware stack.

I think you will need a good processor board like ODROID, Galileo, Raspberry to do the preocessing and then conect it to a microcontroller that will receive instructions to initiate GPIOs for the purposes you have

All Micro-Controllers has ports allowing it to have programmed I/O. the rest of the list you offered are companies or architectures that have been used in microprocessing projects.Pic for instance is a RISK micro-controller, because it's instruction set has been reduced in order to streamline production and speed. PIC's tend to have small word sizes and be used for small control projects normally used low  level language where tight control of input/output is important,

Intel is a manufacturer of a wide range of chips from CISC (Complex instruction set) to RISK including DSP's etc. each processor family has it's own assembly language and word sizes tend to max out at 64 bits.

ARM is the name for a specific family of chips it has a reduced instruction set, but tends to have 32 or 64 bit word sizes.

For mega project i think you need programmable logic controller (PLC).

As my fellow researchers pointed out, microcontrollers have a very broad range and even within the same family lets say PIC u have very different specifications.

So I would suggest you to state your requirements and may be from there we can guide you to which is better for your task.

It is very difficult to advice in this field, because the offer of modern microcontrollers is very wide. As to me, I have been working with microprocessors and microcontrollers since the 70th, and I have changed a lot of different types of them. But I have good experience with ARM families of TI microcontrollers Tiva and Stellaris from the last time. The reasons why I am satisfied with them is the availability of cheep prototype kits, a lot of support literature, and downloadable development software. I used the kits in construction of the school laboratory kits equipped with the graphical display, sigma-delta ADC, DAC, twins of DDSs, PWM outputs, etc., and we use them in our coarses.