if you have a project application in mind you can draw up a requirements specification, e.g. number of digital IO required, ADC inputs, timers, USB, network, etc.
You can then enter your requirements into a product selector such as Microchip's https://www.microchip.com/maps/microcontroller.aspx
do you intend to build your own PCB or use a development board?
Can you let me know your project requirement in detail?
Like CPU, memory, peripherals...
Actually we have many ARM boards which can give you some reference.
You can have a look at www.myirtech.com