Which STM32 development board is suitable for a beginner?
I am also a starter and have been using STM32F407VG-DISCOVERY board over 2 months and it has almost evetything a starter may need. But in my location discovery boards were more expensive than Nucleo boards. I guess a Nucleo board also does the job. However, discovery boards has 4 user LEDs while Nucleo has 1 or 2.

The maximum clock frequency is not a very important issue, just avoid buying a board based on a mcu less than 32kB flash memory. That is because when using stm32cubeide and its automatically created projects, the required flash size was almost 27 28 kB for simple projects.