I meant in a "starting civilization over again" sense, so anything which is easiest given what you have at hand. Core memory seems easier to massproduce without foundries compared to transistors.
But if you have worked your way up to transistors already, the step is probably not very far to start doing chips with many transistors on them, given you already knew this was feasible.
people have built entire computers out of square-loop ferrite (the kind used in core memory) and diodes; ferrite/diode systems were especially popular in the ussr in the late 60s because their transistor fabrication technology was lagging
magamps are still a thing today in some atx power supplies
as i understand it the main problem is that you can't get them to run any faster than a few megahertz
But if you have worked your way up to transistors already, the step is probably not very far to start doing chips with many transistors on them, given you already knew this was feasible.