The topic says it all, Which of these microcontrollers do you think is the best so far in terms of robot development [speed,multi-tasking, ease of use, compactness/size, language support …etc], Please state in the comments why do you like your choice 
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I said other, as I think this is a trick question 
To me the best one is whatever works best for you. As with most things, it is a series of trade offs.
Basic Atom Pros including the Arc32 are a great little micro-controllers. There pretty easy to learn have a fair amount of power and the like, but are limited on program size, multi-tasking…
Arduinos are also nice for smaller projects. I prefer to program in C/C++ so they are a bit more fun for me. However you can do C/C++ with BAPs as well, however Basic Micro does not provide anything as far as libraries are concerned.
Propellers - Played around with them awhile ago. Nice processor, but Spin would takes a bit for me to get used to…
However if your project needs/wants to do more CPU intense things like image processing and the like than you will find that many/most all of the above may not handle it. Then you might find yourself starting to look at some of the smaller Linux boxes, such as the Raspberry Pi or Beagle Bone Black or Odroid. These are the current ones I am playing with. But if you then find your self wanting to do a bunch of 3D image processing, with something like a Kinect sensor, you may find you want a PC as many of the middle ware libraries are only supported on Intel based processors. That is why a few people that I know recently started playing with Intel NUC boards. BUT: all of these systems come with a much much much bigger learning curve!
So again it depends…
I enjoyed working with the PIC controllers. Although i’m not using their full potential in most cases, I like them because they are just chips so you can make your own carrier board for them which can be as small as your design is 
. Arduino is probably my 2nd choice to go since it is same as PIC except that their boards are relatively big