Are you kidding? Kof98 was Uber popular in 98. If you can find the dhp neogeo mailing list archives, 1998 was predominantly a discussion of kof98.
Street Fighter III New Generation was a predominant discussion of 1998 as well. People also believe 3rd Strike was a success for Capcom, and not a nail in the coffin of their games division that they didn't rebound from it until just recently (SFV and games before that were done by 3rd party companies if someone didn't know).
Better how? LB2 & SSII are weapon based, RB2 & MotW each introduced stark modified/completely renewed Garou engines not represented in the KoF series. The lack of reason behind your statement makes it look like you're confusing 'better' with 'different'. The latter is merely a subjective argument.
The op said best fighting game made on the neo geo. Not best KOF game on the Neo Geo. I didn't want to wall of text with reason after reason I thought summing it up quickly would suffice but oh well.
KOF '98 - mostly recycled from KOF '97, shitty backgrounds, almost completely recycled music, game engine is considered the vanilla of KOF based games. Meaning there are other engines that are superior to it.
Garou - Better Graphics, all characters viable, the considered "lowest tier" character is the most popular (Rock)
Last Blade 2 - Better Graphics, the game engine is easy to get into hard to master, has instant kills (apparently they are a big deal),
Real Bout 2 - Better Graphics, Fine Tuned Game engine, managed to update how characters played compared to RBS, Has the biggest roster size for a FF game
Samurai Shodown 2 - Game Engine is regarded as one of the best of all time, instant kills,
Again I love '98UM since they fixed a lot of the problems '98 didn't have the time to do. new mid boss and bosses to choose from, new specials and supers for certain characters making it feel different the '97, added to a lot of the ex versions to make them more complete, added extra music to ex characters or music tracks for certain custom teams making it feel less recycled and more a celebration of KOF so far. The only thing that still sucked was the stages, but I can live w/ that.