For C-programmed microcontrollers:
I started with the PIC18F4550 which has a lot of peripherals.
Then I tried the PIC24FJ64GB002 for a wireless datalogger and it has much more things inside.
Now I am working with a C8051F338 and it's pretty much the same thing, with less peripherals but it is programmed in the same way.
You should start with a basic PIC like the PIC16F687 and learn how to write good C programs.
Also stick with one compiler: for example SDCC and learn all the syntax of interrupts and so. Don't mix up compilers...
For other programming languages:
I bought a Parallax Propeller P8X32A but haven't programmed it (programs are written in asm or Spin code).
They also sell the BASIC Stamp which is programmed in qBasic, or the Javelin Stamp programmed in Java.
I just love C and think that the PIC microcontrollers are more flexible.