I tested a bunch of fighters (including all KoF titles) with the Unibios for the character unlock cheats to see what was available, almost everything I threw at it had all characters playable which is really amazing (hard to stress how incredible a feature this is).
The only two that I found where the bosses don't seem to be playable are King of Fighters 99 (Krizalid only) and King of Fighters 2000 which has no character unlock cheats at all. I guess that game got the least love. In AES only Kula is playable but no Zero. Is KoF2000 one of your least favorites Razoola?
Really not essential but as a huge King of Fighters 98 fan I would have found it fun if Omega Rugal were selectable through the regular Start + cursor over Rugal code in MVS via a unibios cheat, but unfortunately that's not there.
Last Blade also has a mistake in the Unibios on the MVS side, there is an option for Unlocking Musashi (I think his name is mispelt too) and Kagami, but apparently Kagami is not playable in MVS. In AES mode this cheat works properly and both characters are selectable.
For a short run down of what I tried here's what's unlockable:
King of Fighters 94: Rugal (in mvs and aes) --- All characters playable
King of Fighters 95: Saisyu and Omega Rugal (in mvs and aes) --- All characters playable
King of Fighters 96: Chizuru and Goenitz (in mvs and aes) -- All characters playable
King of Fighters 97: All of them, including Orochi (in mvs and aes) --- All characters playable
King of Fighters 98:
No character cheats --- All characters playable in AES
King of Fighters 99: Kyo and Iori (in mvs and aes) --- Krizalid not playable
King of Fighters 2000:
No character cheats --- Zero not playable
King of Fighters 2001: Zero, Igniz playable but with issues --- All characters playable (some issues)
King of Fighters 2002: Omega Rugal (in mvs and aes) --- All characters playable
King of Fighters 2003: All of them including Adel and Mukai (in mvs and aes) --- All characters playable
Last Blade: Musashi in MVS, Musashi and Kagami in AES --- All characters playable in AES mode
Last Blade 2: Kouryu and Hagure (in mvs and aes) --- All characters playable
Garou Mark of the Wolves:
No character cheats --- All characters playable in AES
Samurai Shodown: Amakusa playable in both mvs and aes --- All characters playable
Samurai Shodown II: Mizuki and Kuroko playable (in mvs and aes) --- All characters playable
Samurai Shodwon IV: Zankuro playable (in mvs and aes) --- All characters playable
Real Bout Fatal Fury Special:
No character cheats --- Nightmare Geese not playable (except via manual codes)
Real Bout Fatal Fury 2:
No character cheats --- Alfred not playable
Ninja Masters: Ranmaru and Nobunaga playable (in mvs and aes) --- All characters playable
Sengoku 3: Byakki and Okuni playable on character select --- All characters playable from start of game
Wish I had some more to test (such as SVC Chaos). Love that Razoola took the time to create these awesome character unlock cheats, now one of my favorite aspects of the Unibios. I was stunned when I first tried it on KoF97 and found Orochi playable. As a younger teenager that was like a dream of mine, and unfortunately was only possible on the Playstation port and Neo Geo Pocket in R1. Now we have some other real stunners, like KoF2003 allowing the use of all the hidden characters. It's amazing.
EDIT: I added Sengoku 3, despite not being a fighter, since it has character unlocks.