I think a huge piece of the experience is lost to time. KOF94 was a pretty groundbreaking game. Cross over characters like that, at that scale, had never really been done before. Kyo wasn't even focused on much, Iori wasn't in it. It was all about Fatal Fury versus Art of Fighting with other SNK characters thrown in. I mean the Ikari warriors in a fighting game? That's honestly pretty wild.
I didn't play KOF94 until 1997. I imagine SNK fans who had put hours into FF and AOF up to that point must have shit a brick when they saw KOF94's intro.
So for sure, KOF94 has to be included in the essentials. And then probably 98, 13, and 15. Although I do hesitate to include anything made from SNKP on.