Not the cheapest perhaps, but I recently used Ambiq's Apollo for a project. It's an ultra low power ARM MCU. I found it very nice to use mainly because Ambiq only has two models at this point, which makes their libraries very clean.
The other advantage is that its I2C slave core operates autonomously- it provides a shared register space. It means that a master can write a bunch of stuff to the shared memory without waking up the CPU during each transaction (which usually involves clock stretching).
The other advantage is that its I2C slave core operates autonomously- it provides a shared register space. It means that a master can write a bunch of stuff to the shared memory without waking up the CPU during each transaction (which usually involves clock stretching).