I'm a big fan of the MSP430, which is the only ultra-low-end microcontroller that's not Harvard architecture. That is, code and RAM live in the same address space, which lets you load code at runtime without needing to fiddle about with reflashing.

This allows you to port real operating systems to it:

http://cowlark.com/2015-10-27-fuzix

It's also a really elegant architecture which is a pleasure to write assembly for, and it consumes absurdly small amounts of power.