As much as I like KOF FF to me is a better series cause a lot of the games feel different from one another. FFS doesn't feel like RB2 and MOTW doesn't feel like them either.
Thought I would elaborate. Sorry if it's pointless. With most fighting games you have one character that never really changes through updates. I call him the control character. The most famous one is Ryu. Now if you play Ryu in different games he tends to always be similar. If he plays the same and has the same matchups, then the new game is just an update of the last game. for example from SF to SFII he is drastically different all the updates of SFII he's the same except for a new move or two, but you approach using him the same. SFA he changed again SFA2 and SFA3 he's the same. SFIII another drastic change SSFIIIGA and 3rdStrike he just got extra moves but plays the same.
KOF's control character is Iori. Iori plays the same throughout minus the new update powerless version. 95 and 96 play a little differently, but from 96 to 03 he might get a move or minus a move but Iori is Iori. He works the same way for the most part and in XI the game engine really changed , not Iori.
Samurai Shodown it's Haohmaru 1 and 2 play slightly different but close. 3 and on though are the same game with updates to it.
Fatal Fury's control character is Terry Bogard. He changed drastically from 1 to 2. Was the same in Special, drastic again in 3 similar in RB, another big change in RBS mostly same in RB2 and drastic in MOTW. Considering FFS, RBS, RB2, and MOTW are stilled played a lot that makes me think FF is superior.