Now that I think about it, I do have a few favorites when it comes to my favorite ryona girl. its hard to limit myself to just one, though, so I'll just list my faves.
Street Fighter: Sakura, Ibuki, Chun-Li (although I'm nearly open to almost all SF girls throughout the series. I think the only 3 I don't go that route with is Viper, Sharon and R. Mika [was never a fan of Mika to begin with]). Street Fighter IV just amplified my fascination for them even more. Sakura's a special case since she's my all-time favorite fighting game girl all-around and my best to play as.
Tekken: Lili, Xiaoyu & Asuka (ironically, my three main characters to play as in the first place. I also have a spot for Alisa as well).
Soul Calibur: Talim, Amy, Seong-Mina, Cassanda and Xianghua (same reasons as Tekken. Both series' recent games having huge customizable outfit options don't help in the slightest.)
Rumble Roses: Reiko and Becky, although I do sometimes do Aigle, Makoto and Noble Rose as well.
Dead or Alive: Kasumi, Ayane, Lei-Fang, Hitomi & Koroko. (DoA2U and 4 for these girls, no doubt about it. I'm always a fan of the types of 'casual' costumes they wear, and I'm not talking about the skimpy ones.)
King of Fighters: Yuri, Kula, Athena (depending on the game for Athena, sometimes she's just annoying, though...)
Samurai Shodown: Nakoruru, Rimururu (although I've only played Sen, that game is more than enough to keep me satisfied for these two.)
Bloody Roar: Uriko, Alice (the 3rd game's definately my favor for some reason, although Primal Fury and 2 was great too.)