I collected together Doomworld's Top 100 Most Memorable Maps and started playing through the list, from #100: https://doomwiki.org/wiki/Top_100_Most_Memorable_Maps
This is a bit of an undertaking, and I've played somewhere between 10-25% of them already, but it'll be good for filling in a lot of gaps in my knowledge, and food for my level design brain of course.