Almost 20 years I was designing product with 8 bit microcontrollers since they were the more cost effective option at the time. They had integrated memories which was less common in 32 bit microcontrollers. By today’s standards the 8 bits parts can be a respectable value but their compute and memory limit are too constraining for the broader functionality common in today’s developments.
In the realm of 32 bit microcontrollers the ARM Cortex M series has achieved dominance in the market. The number of proprietary core offerings continues to shrink as they are displaced by ARM. There are a number of advantages. Knowledge gained about an ARM Cortex M based part can be transferred to parts from other manufacturers. ARM is continually growing their development resources and libraries that are common to all Cortex M parts and development tools can be common.
The absolute lowest cost microcontrollers are still 8 bit core but it still may be wise to use an ARM Cortex core to reduce development time. The lowest cost 8 bit cores might be $0.25 while the lowest cost ARM Cortex is $0.35. For that extra $0.10 you will get extra compute and memory resources that may make you development faster and enable additional product features. For $1 your memory and compute resource will likely support the use of an RTOS which could further speed you development.
There are many options in Cortex M microcontrollers. There are STM32 from ST, ATSAM from Microchip, MSP432 from TI, and LPC from NXP and many others. I have the most experience with STM32 because they had the best library support for fast product development when the family was introduced in 2007. They maintain great library support though many manufactures have caught up in that respect. Nordic, for example, has more extensive libraries supporting Bluetooth connectivity.
If you pick a Cortex M based microcontroller for your product design you will have the broadest support in libraries and tools and most peripheral options from the greatest number of microcontroller manufacturers.