I think there are free PIC assemblers but no free C compilers. All the tools needed to actually load code onto the chip are either proprietary or don't work with the modern programmers.

No great loss in my opinion. PICs are old and slow, and the C programming experience is much worse than an STM32.

Interesting re: pics. That might be why people I know who use them used assembler. I did use them once, and used assembler myself, ages ago.

I'd say avr8, msp430 and stm32 cover the full range.

avr8 (atmega/attiny) is 8bit, very easy to understand, has excellent open ecosystem, the go-to if you don't need the other two. There's fancy new xmega stuff, but at that point, you'd look at stm32.

msp430 is 16bit, smaller rom/ram sizes but very low-power and excellent open ecosystem.

stm32 is 32bit, Cortex-M0+ based in current generation,

pic? attiny trounces pic12, atmega trounces pic16/18, msp430 trounces pic24 and stm32 trounces pic32.

The STM32 is even better than that because the family scales up and down from M0 to M4 or higher without much of a change.

Plus the free libraries that ST has made available are magical. Just - incredibly good. Best embedded C experience I've ever had. Yes using the chip in a more 'bare metal' scenario can be challenging (set 4 registers in the proper order with obtuse values to route the clock signal properly), but it's usually not necessary since the provided libs are so cohesive and comprehensive.