My $0.02. Just an opinion.
The even numbered KOFs tend to be the best ones, starting with KOF'96. The odd numbered ones tend to be the most broken in gameplay terms.
'94 - Easy infinites and re-dizzies
'95 - Rediculous damage and infinites
'96 - Set the tone for every KOF that followed. Included the infamous Boss team in addition to introducing Leona, Vice, Mature, Goenitz, etc. One of the best in the series.
'97 - Horrid, horrid, presentation. The art style used is terrible and the lack of music in almost all of the fights is definitely not a good thing. Not to mention this game has a slew of infinites.
'98 - A much improved version of KOF97. Many gameplay issues have been fixed. The presentation was improved. Brought back Takuma, Heidern, Saisyu, and the Sports Team. THE BEST in the series.
'99 - The only odd numbered game in the series that is actually good in all areas. This game is the only exception to the rule that odd numbered KOFs suck. Great presentation, lush backgrounds, all new soundtrack, and great new characters.
'00 - KOF99 on steroids. Everything was jacked up for this one including the roster, number of strikers, and gameplay. The only thing '99 has over this one are the backgrounds.
'01 - One of the worst in the entire series imo. Gameplay is solid but it does not compensate for the disgusting palette used throughout the game or the horrid art and music.
'02 - The second best game in the series. All of the most popular teams have returned. Comes close to '98 but '98's nostalgia wins.
'03 - Broken. Broken, broken, broken, broken, broken game. So many infinites it ruins the game. Hit velocities have been greatly increased and hit pauses have been reduced. The result is a faster game, but one that does not really feel like the KOFs of old. To its credit the backgrounds are great and the story is interesting but I suspect '04 will crap all over it.