What processors or microcontrollers are considered 4-bit? 8-bit? 16-bit? 24-bit? 32-bit? Which have you used in each size group? Which is your favorite or hate?

Processor "bit-width" generally refers to the native data-path/register width. Representative examples by size:

• 4-bit: Intel 4004 (the first commercial microprocessor) and various early/very-low-cost calculator and appliance microcontrollers.
• 8-bit: Intel 8051 family, Atmel AVR, Microchip PIC (mid-range), Zilog Z80, MOS 6502, Motorola 68HC05/68HC08. Still ubiquitous in cost-sensitive embedded designs.
• 16-bit: TI MSP430, Intel 8086/80286, Microchip PIC24/dsPIC, Motorola 68HC16.
• 24-bit: Found mainly in DSPs, e.g. the Motorola/Freescale 56000 (56K) audio DSP family.
• 32-bit: ARM Cortex-M (microcontrollers) and Cortex-A (application processors), x86, MIPS, PowerPC, and RISC-V (RV32). These dominate higher-performance embedded and general-purpose computing; 64-bit variants (ARMv8-A AArch64 Cortex-A cores, x86-64, RV64) extend the same families.