The best choice is the processor which you are able to use! - by far the most popular choice is the MicroChip PIC - due to it's ease of use and cheap price.
A choice often used in robots is the 68H11 series, but this is a lot more expensive and difficult to use than the PIC's, more like a micro-processor than a micro-controller. This appears to have become popular from the universities, where they have the resources to use more expensive gear?.
But many people use PIC's or AVR's, if you are already familiar with a processor it makes sense to stick with it - if not, you might try using either the PIC or the AVR. There's a LOT more help about for PIC's, basically because they are more popular, and have been about for a great deal longer - but the AVR is a fine processor as well.