Atmega328 in the simple form to get next prototyping up and running quickly on breadboard, veroboard etc. No more rewiring the breadboard for a minimal circuit.
Build in programming header: Standard ISP programming header.
Design small enough to be plug-in CPU board for another project.
ZIF socket - the AVR chip can be transfer to other project easily.
Crystal socket - allow the use of different clock settings to work on different voltage range.
A programmer is needed. Buy one or turn your Arduino board into ISP programmer.