The STM32F103RET6: This is a workhorse MCU for control, instrumentation, and communication tasks where the peripheral mix and memory size hit a sweet spot between cost and capability.
72 MHz core — timing and throughput
The 72 MHz clock is the maximum for this Cortex-M3 implementation.
512 KB of Flash is enough for a full-featured firmware stack: a real-time OS, USB stack, CANopen or Modbus library, plus application logic. The 64 KB SRAM supports moderate-sized data buffers (e.g., 8–16 KB for USB endpoints, a few KB for CAN message queues) without external memory. If your application needs larger frame buffers or extensive data logging, the 64 KB RAM may be the tighter constraint — check your heap and stack budget early.
Peripheral set — CAN, USB, and serial interfaces
The 51 GPIOs give plenty of headroom for local sensors, switches, and indicators alongside the serial interfaces.