How to make a Microcontroller choice?
There are dozens of microcontrollers that fit that bill. Your choice may depend on some of the following criteria:  


price  
availability, especially if you need them in production quantities and for long-time production. In that case choose a supplier who doesn't often outphase controllers.  
development tools. That's both software and hardware. These days many controllers have on-chip emulation, so that you don't need expensive emulators anymore. Some manufacturers provide a development environment that allows you develop C programs, for others you need third-party tools which may be costly. Many of the more expensive IDEs have a limited version for free or for a small fee. Usually limits the amount of code it can compile, though sometimes the limit can be as high as 32kbytes, which may be more than enough for your needs.  
ease of use. Some microcontrollers are so versatile that it seems they can solve any problem, but have a long learning curve. You don't want to study a 2000 pages user manual to write a "hello world" program.  
good (FAE) support. That will be more important for professionals, distributors don't care about hobbyists.  
package. You will need to find a package that is small enough to fit your needs while being big enough to be able to solder.


Your final choice will depend on how important each of these parameters is for you.