The really cool thing of the KOF series is each chapter is unique, and no chapter make its former 'useless'.
KOF94 - Awesome background, a someway 'extreme' style in color and drawing srpite, the first team experiment.
KOF95 - The 'more of the same' of the 94, with biased gameplay and appela, Team Edit now.
KOF96 - Where the KOF-style begun- Very extrem gameplay totally 'short-range' oriented, awesome graphics and sound.
KOF97 - KOF96 evolution, someway 'less extreme', taking bit of new and bits of old. Advance mode introduced, interesting experimental sound concept.
KOF98 - A dream match which represent the biased and perfectioned version of the former 97. Loads of stuff for any fan and player.
KOF99 - The new revolution. New system, new characters, new starrings. Also new 'techno' atmosphere. Super cool.
KOF2000 - KOF99 extended and retouched. New cool characters introduced, striker system at his max.
KOF2001 - New great integration of strikers and fightersa, maybe the final point of the 99 playability?
My fav remains 96 and 2001, but it's a matter of tastes after all...