Warning: This is a little long, so I apologize beforehand.
I don't know that ANY of the KOF games 'suck'. They're all worthy games in my opinion.
But, even after saying that, one of the games is going to be the best of that series and one of them is going to be the worst. The only way that I could pick which is the 'worst' is to really look at it from an achievement standpoint. In other words, which one 'underachieved', or fell shortest of its expectations. Game by game:
'94: Matched most of its expectations. Gameplay was crisp, fresh and new and controlled very well. The idea of a fighting game with many SNK titles while at the same time adding new characters was high concept and mostly worked. Graphics, animations, backgrounds and BGM top notch. Second stage Rugal was NOT an easy boss to beat, to SNK's credit. Going back and playing, it has more than stood the test of time. Other game companies should be proud if their games today are as good as KOF '94.
'95: Tweaked gameplay, redrew some characters, added edit mode. Soundtrack was a bit weaker than the last but the backgrounds were just as good. Added a cancel attack to the dodging. Introduced Iori and his rivalry with Kyo, gave us the Orochi storyline, removed sports team and added rivals team. Bosses playable with a code (thank you, SNK). Damage WAY too high, but overall a worthy successor.
'96: Changed game engine dramatically; added rolling, dashing, guard crush. Removed dodging (for most characters) and 'hopping'. Took away most full screen projectiles in favor of more intense, close-in fighting . Super desperation moves added. Game has a largely untested feel to it, but doesn't hurt overall experience. Character graphics redrawn and look a little smaller. BGM no better than '95, but backgrounds are generally awesome. Added boss team (good), but lost Eiji and Billy (bad).
'97: Gameplay issues from '96 tweaked. Play is more refined; many old issues solved but new issues come up. Additon of 'Advanced' and 'Extra' fighting modes is welcome; play in KOF 94-95 style or in KOF 96 style with modification of building DM stocks as opposed to charging power meter. Backgrounds are bland, boring and uninspired except for a couple of stages and BGM is nonexistent except for SNK marquee characters (WHY!!!!??!?!). Oddly enough, original soundtrack is two discs! There IS a lot of music in the game, but you'd never know from playing it:P! Second lamest boss in all of KOF, boss team gone (bad), but replaced with some new charas, most notably NFT, Blue Mary, Yamazaki and a returning Billy. Side note: Where's Eiji? Character is too cool to only be playable in two games (Zantetsu in Last Blade doesn't count).
'98: Dream Match, Dream Game. As close to perfect as KOF is going to get. Gameplay is the best of the series. Great backgrounds overall. Great soundtrack. Best character selection of any KOF, but still no boss team characters or Eiji, which would have been preferable to the returning American sports team. The return of Rugal, redrawn graphics (thank you, SNK), Heidern and Saishu. Hidden versions of many of the SNK marquee characters with full screen projectiles and different moves. Variety is appreciated. Lack of story mode doesn't hurt game's appeal.
'99: Four character teams and Striker mode is added. Armor and Burning modes added. Gameplay tweaked, characters given some new and interesting variatons of gameplay, and control is a little more unforgiving. Story focus is shifted to K', the coolest character to come from the KOF series ever, and the Nests cartel. Other welcome additions (Juhn, perfect Kyo) are counterbalanced by cheese and stupidity (Whip, Bao). Most unwelcome change for me is in Robert Garcia's gameplay and animations. What happened with him? Backgrounds are very good and BGM is above average with a few standouts. A good game, but by now KOF's becoming a bit long in the tooth. Series turning into a 'fans only' love affair.
2K: Gameplay adjusted slightly. Controls a bit more forgiving. Extra and Hidden/Maniac strikers added for flavor, but these aesthetic touches don't impact the game much. However, infinites involving certain strikers are definitely a problem. Some interesting team variations, some cool new characters added. Hinako is not one of them
, and her presence almost singlehandedly ruins the ladies team (Xiangfe would have been better); Mai and Yuri do NOT belong in high school comedy anime/manga fare:P. Backgrounds are okay, if not remarkable, but the BGM is fantastic, ranging from techno progressive (example: K' music) to a fusion between jazz and dance (example: Benimaru team music). Zero....in best Simpons 'comic book guy' voice: WORST KOF BOSS EVERRRR!!!. Boss so lame it defies description. Well animated but poor conceptually. Only the addition of Kula Diamond ('anti-K'') can counterbalance it, and barely does so. Dramatic storyline ramifications to KOF universe as Southtown is destroyed by orbital laser, thus paving the way for construction of Southtown Second, the city in Mark Of The Wolves. Will be interesting to see if K' will play differently in the future now that he's not wearing 'the glove'. Overall, the game is a letdown due to the quality not measuring up to the hype, but by no means is it a bad game. Still better than most 2D fighters out there. Series needs at least one more game to end Nests Cartel storyline and wrap up all other plotlines.
So, based on this rather long winded examination of the KOF series, the only one that looks mostly negative is '97.
Therefore, KOF '97, for the reasons I've given, is my pick for the 'worst' of the KOF series. That is, the most 'underachieving'.
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Taiso
'For great justice take off every ZIG'