Important specs while selecting MCU

1. Memory architecture: Von-neumann or Harvard or Modified Harvard
2. Instruction set architecture: RISC or CISC
3. Operating frequency
4. CPU operating modes
5. Coremark benchmarking results of MCU. Or results like matrix multiplication/inverse calculation, FFT etc. related benchmarking is good to judge MCU.
6. Application category based selection: Ultra low power, Mainstream, High performance (as in case of ST)
7. Peripheral available on MCU
8. Number of Timers: Basic, Genereal purpose and High resoluation
9. Number of external interrupt lines.
10. Number of internal interrupt lines.
11. DMA necessary when large data transfers are important in application.
12. CRC and MPU (memory protection unit):
1. CRC is used while compiling teh binary and writing data to Flash. Its is then decoded using the same CRC to verify the read instruction is correct, inside CPU
2. MPU is used mostly in RTOS level applications, where we need isolation between kernel area and user domain. This protects the kernel memory area from user code intervention
13. FPU (Floating point unit): Its is necessary when there are many floating point multiplications and divisions in the code. This makes the code execution very fast, as compared to the software FPU implemetation (aebi_vfpu())
14. ADC: High sampling rate, Effective number of bits (ENOB), Resolution. Also look for internal temperature compensation availability as it helps if compensation of temperature drifts in the application. 
15. Communication peripherals: I2C, SPI, UART
16. Analog peripheral on chip: programmable gain OPAMP, Analog COMPARATORs. These are necessary for applications where they play role in reducing cost of products, by decreasing cost of external components required.
17. Power consumption in various modes. This is particulary important when you are dealing with portable device applications like electronic handheld device, smart watch etc.