Low level is mostly not about processing core even if you are using assembly language. Yes, you can reach guru level in assembly language of some target platform but to do something really useful at the lower level you need to know and be able to use the peripheral.
And the hardest things are in peripheral.
Any modern microcontroller it will give you the opportunity to learn a lot of useful things about peripheral devices. You can start with any really good modern 8-bits micro like Microchip's 2nd generation of ATTiny, so you'll have in your hands a lot of very powerful interesting smart peripherals: hardware event system, small programmable logic, different timers, good ADC etc.
The only rational additional consideration here is that your target platform should be popular, well documented and supported by the manufacturer.
Then will be time for some Cortex-M0 device with DMA.
And the hardest things are in peripheral.
Any modern microcontroller it will give you the opportunity to learn a lot of useful things about peripheral devices. You can start with any really good modern 8-bits micro like Microchip's 2nd generation of ATTiny, so you'll have in your hands a lot of very powerful interesting smart peripherals: hardware event system, small programmable logic, different timers, good ADC etc.
The only rational additional consideration here is that your target platform should be popular, well documented and supported by the manufacturer.
Then will be time for some Cortex-M0 device with DMA.
Then you'll decide where to go further :)