Been playing KOF avidly since 1994, so here is my top 5 list - I like all KOF games, though, and own most in the series in original format:
98 (One of the most well rounded fighters ever - and one of my top 3 favorite fighting games overall)
95 (I have a history with this game, and it is 95 that got me to originally purchase my AES in the late 90s)
2K (I love the update to the Striker system mechanics, as well as all the "extra strikers" that celebrate SNK history)
99 (Mostly just from an artistic perspective - The overall presentation, music, and sprite work was awesome)
NeoWave (a better version of 2K2 imo - there is so much I dislike about 2K2, mostly in presentation)
I haven't played much modern KOF after 13, but what I have played of 14 I found kinda meh. I think I'm just old and jaded and prefer classic KOF because nostalgia.