Not merely System 6, but System 6.08L, specifically!
Some hooks to the features soon to come in System 7, but w/ all the insanely ugly & stupid desktop patterns stripped out, & other unnecessary system resources as well. Small footprint, as fast as it comes. Load it up in a Powerbook 100, even w/ the base 2MB of memory, you can still allocate enough space for 6.08L to a ram disk [no need for a disk cache; trim that to zero! System 6' disk cach mgmt. sucked anyway...] and have enough ram left over to run MacWrite II.
Best of the System 6, & so unknown that it's still mostly a rumour after a decade & 1/2. But I was there, I used it, & still miss the rockin' performance that running completely out of ram gave to the otherwise sedate PB100. Of course, by today's standards that was still dog slow, but at the time it was the best bang for the buck available for mobile production [no internet to distract you from getting things done is still sometimes a valuable feature!