Sorry. There really isn't a good answer to this question as any embedded developer with a modicum of experience knows. In some cases there might be a specific device or family of devices that is optimum for a particular application but in general there are usually a wide range of products from numerous vendors that might fill
If you need to answer the question then check out Robert Cravotta's new Embedded Insights website. It is one of a number of sites that provide a searchable database of vendor information about microprocessors and microcontrollers. Another is available from GruntWare that I reviewed in Gopher: Find Me A Microcontroller.
What actually prompted this article was reading Robert's What matters most when choosing an embedded processor? His answer was turning to the question over to the readers. The responses highlight the range of requirements that developers encounter.
One issue he brings up is one I agree is very high on the checklist: development software. Pennies are important if a product will be shipping in the millions or billions but processor cost is less of an issue as the numbers go down. In this case, the processor choice can be very important but in most cases the cost of software development is likely to outweigh most issues including processor cost.
One thing I have been doing for the past decade with Electronic Design is to check out development and eval kits. The hardware has improved over time but develpoment boards for a particular class of systems has changed little over time. On the other hand, the amount and quality of the software that accompanies these kits has changed radically for the better.
Kits that have no bundled software are extremely rare. There are still a significant number that come with crippleware but the majority come with full blown tools. In some cases the latter may have a limit such as the amount of code it can generate but it is often within the limits of the kit. For example, a microcontroller may have 32Kbytes of flash program memory but the top end chip may have megabytes. The important thing is that a developer can take advantage of the kit to do their evaluation or prototypinng.
The other change is the amount of open source software that is part of the mix. This is sometimes the reason more software is included but it does mean that the vendor has done the integration needed for a good user experience. It is also the reason there is more commonality among vendors at least when it comes to major platforms like the Eclipse IDE and Linux.
The upside to this is developers get a better chance to see if a platform will be viable from a hardware and software standpoint using relatively inexpensive kits. Of course cost is still relative. I have seen some kits that run over $500 but for what they provide they are actually a bargain. Not every platform is easily presented for under $20.
One trend that has become a bit more noticeable is a move to more commonality within some vendors development product lines instead of each kit being a one off platform. Freescale's Tower system (Fig. 1) is one of the more consistent approaches using low cost boards and PCI Express connectors for its prototyping and eval system. Outfits like Microchip have had similar semi-standard plug in or expansion systems like its PICtail and Explorer (Fig. 2) boards. Other vendors like Rabbit Semiconductor implement their entire line in the form of standard modules like the MiniCore RCM5600W (Fig.3). Of course, some vendors like Rabbit are looking towards module sales while others are looking for chip sales.