suggestion
In my opinion, at first, it doesn´t matter the micro you choose. Like PIC's family, 8051 family, st60, etc. The most important is that you understand microcontrollers architecture. Choose a micro, find docs about this micro, and attemp to understand it, a first will be hard because asm instrucctions may be a bit problem. ypu have to profundize in this micro and unterstand everything on it. In a bit time yo will have the necessary knwoledge of it. Then get a emulator and a compiler of this micro to run application and probe your knowledge. Step by step you'll undertand the micro and you be a little independient from the micro, using a C-Compiler, Basic compiler, etc. At the end you'll be totally independient form the micro ( you will only use low level routines to make you low level drivers, like acces a port, a timer etc). and you can choose another micros.
Start whit PIC's family or 8051 falimy will help you. In thier basic versions, they are very esay to understand.