Barthood could be the penultimate episode I suppose, as far as chronological placement goes.
I just remembered that Lisa is in a relationship with Nelson by the end of Barthood, but married to Milhouse by the start of “Holidays…”, we still don’t have a clear idea of when Lisa finally chose to have a family with Milhouse in between that in the series.
I know there was one modern series episode where Lisa finally kissed Milhouse, but that’s when they were still kids.