If you ask which is the best microcontroller today, difficult to answer. all families have more and less. if you want to learn, you should choose a microcontroller that is available at your place, availability of resource and software development. For the availability of software and resource, I suggest ATMEL more than the others (i'm sorry for lovers of others, it's my subjective thought, because i was an ATMEL's user).
I suggest you learn a little about assembly language just to know the basics and then immediately switch to C, because you will be able to realize your ideas and designs faster.