Every KOF is great.
Even darkhorses in the series like 99 which offers something intuitive during it's introduction.
94 - Introduction of 3 vs 3. Nuff said.
95 - Super Big Damage. Team Edit. Introduction of Iori.
96 - Huge Gameplay Changes. Boss Team!
97 - Dual Modes, Lotsa New KOF-ers.
98 - Enhanced Dual Modes, Extreme Balancing.
99 - Introduction of Strikers. Roll Back Disabled.
00 - Infinites Galore!
01 - Superior Depth and Combo Creativity.
02 - Basically an enhanced 98 (gameplay wise) except with lousier music and backgrounds.
03 - Drastic gameplay changes once again. Tagging system. FASTER overall.
I'm a KOF whore. I bought a Neo-CD for KOF 95 and years on a Neo-AES for KOF 98 (cause I can't stand the loading no more)
I can't say how I'll be able to leave out any KOF in my collection but if i have to own only 1, it's 98 i'm damn sure.