What micro-controllers are suitable for a closed-loop DC Motor?
Assuming a brushed DC motor, if you're planning on tachometer feedback a quadrature encoder peripheral would be a good thing to have. On-chip ADC may or may not be adequate for the analog inputs- current and maybe rail voltage for feed-forward.  

Speed control, as opposed to position control, generally is pretty undemanding and I suspect most 8-bit, 16-bit or 32-bit MCUs with PWM and quadrature encoder peripheral would be acceptable in most situations. 

Using a 32-bit micro would allow more sophisticated control algorithms or easy implementation of simpler ones. You can get a Cortex M4 with single precision FPU for only a few dollars these days so that might be a logical way to proceed- though the complexity of firmware development is an order of magnitude above something like PIC you don't need to worry much about execution time and could probably use an RTOS if that's your bent. Although it's possible to buy processors with (relatively) prototyping-friendly 64-lead LQFP packages, an evaluation board would be a lot easier if you're not comfortable with that kind of thing, then only the power devices would need a PCB.