EDITED to include KOF Neowave.
Ranking the KOF games is always hard for me to do, but here goes. I'm not going to count The King of Fighters Maximum Impact, and I haven't played (only seen) The King of Fighters Neowave (comes out within the next two weeks on PS2!), so I can't place that yet, but I suspect it will end up replacing one of the games in my top three picks, likely 2002. 2003 will undoubtedly be replaced by KOF XI when that releases. I'm ranking these games more on how much fun I had with them at the time of release and/or how much play time I put into them even if I'm playing a lower-ranked game more at the present time.
1. The King of Fighters '98:
I don't play this one as much anymore; KOF 2002,2003, and Neowave get all my play time, but this was easily the high point in the KOF series. Huge roster for the time, game play that improved on the already awesome KOF '97, alternate versions of characters with different move sets, great visuals (easily the most colorful of all the KOF games), music, etc. This one has it all. Without a doubt one of the best fighting games ever made.
2. The King of Fighters Neowave:
(pic is from the arcade version; need a better pic to show off the detail!)
Neowave may have "rehash" written all over it, but when the game is a rehash of KOF 2002, that's not a bad thing. Although the arcade version of Neowave had some questionable roster omissions (some of which ended up as hidden characters), the PS2 version adds in five more fighters, giving the game everyone from 2002 save for K9999, plus Saishyu, Jhun, King, and Shingo. Even the game's one "new" character, the boss, the younger version of Geese Howard, is taken right from Art of Fighting 2. But Neowave breaks the game play system into three separate styles, including one that allows for Just Defends a la Garou Mark of The Wolves, making the game play even more in-depth than ever before. Neowave's backgrounds tend to be a little on the empty side, but there's no denying that these are the best backgrounds ever seen in a KOF game to date, and the filtering used on the sprites gives the game an incredibly clean, vibrant look. The music, a fusion of jazz and techno, is more like the soundtracks found in the older KOF games, and sounds far better than anything in KOF 2001-2003.
3. The King of Fighters 2002:
Also known as "KOF '98 Part 2." Another dream match game that was likely released only because they had no idea where they were going to take the series after the overall dismal KOF 2001. This is the game that basically replaced KOF '98 for me, but I still have to go with '98 over 2002 simply because the overall package is better; both play great, but I prefer the look and sound of '98. Some bonus points go to the home ports for adding more fighters, most notably for bringing back the strangely-missing KOF staple characters King and Shingo, but the game play is not quite the same as the original version.
4. The King of Fighters 2003:
It's tough for me to decide whether or not this or 2002 should be in second place, and while I'm almost always a fan of change, and I love the new battle system introduced in 2003, it still needs some work. I was never a fan of the striker-based game play of KOF '99-2001; the way 2003 plays is the way all of those games should have played as well. I also should note that I much prefer the PS2 version of this game over the original; the new/redone stages and arranged soundtrack, combined with new modes of play including the classic 3-on-3 mode, make it the best home version of a KOF game ever released.
5. The King of Fighters '97:
KOF '96 was the first time in the series' history that game play was noticeably changed over previous versions, yet there was something that felt missing. KOF '97 solved this problem and also introduced some great new characters including bringing in more Real Bout characters (always a good thing); the only thing about it I never really liked was the attempt at going "Samurai Shodown" in the sound department by using ambient/background noise instead of music; only a handful of characters got music. I went absolutely NUTS over this game when it first came out, so you can imagine my reaction to KOF '98 when it came out and was even better!
6. The King of Fighters '95:
The best of the "old school" KOF games. Added the much-needed team edit feature, more moves per character, introduced Kyo's rival Iori, and just felt like an overall more complete game compared to the original.
7. The King of Fighters '96:
Loved it, but there were some things about KOF '95 that I preferred over this version of the game. KOF '96 still has the best background in the series with the "boss team" stage, I really would have liked to have seen Geese (finally returned as a bonus character in KOF 2002 PS2/Xbox), Krauser, and Mr. Big appear in more KOF games.
8. The King of Fighters 2000:
The striker-based KOF games are without a doubt my least favorite out of the series as far as game play goes, and I actually prefer 2001's game play over 2000's, but this one introduced some more great character designs, and I did like seeing all the extra SNK characters showing up as strikers.
9. The King of Fighters '99:
Loved the new characters/sprites, loved the look of the game, but just wasn't feeling the whole armor/counter mode and striker game play features. The lack of characters as well as moves per character compared to KOF '98 was also disappointing. This game felt very similar to '96 in that it was obvious SNK was trying something new, and that the follow-up game would likely expand the roster, add more moves, and flesh out the newer game play features.
10. The King of Fighters '94:
I actually considered ranking this one last as I usually do, but I've been playing a good bit of KOF '94 Rebout lately.
The original game was an awesome idea, but feels very dated now. I never understood why they didn't have team edit as a feature from the start.
11. The King of Fighters 2001:
Game play is the most important part of any fighting game, and I did say that I prefer the game play of 2001 over the other two striker KOF games, but the look, sound, and overall feel of this game was just so dull that it made me go back and play the older games instead. I did like Angel and K9999 quite a lot, though.