Hmm, tough one. 'Sexiest game girl' is easy, but as an actual subject of ryona? That requires some more thought.
Personally...whilst she's still my number-one girl for, um, everything, Nina Williams' voice in recent appearances has never been quite as arousing as her Tekken 3/Tekken Tag Tournament vocals. Every single moan of pain makes her sound like she's moving closer and closer to a major orgasm, and one of her K.O. wails (if you've heard it, you'll know what I mean) is basically an audio climax. Her voice is still sexy in Tekkens 5 and 6, but she doesn't sound as hot when she's hurt.
Also in the Tekken series, I'd nominate Lili. Now...I don't play as her, and I find her personality and most of her lines immensely irritating. However, I think these factors actually work in her favour, as I actually want to see her get beaten up viciously. And whatever her voice actress lacked in dramatic chops, she made up for with some spirited yelps of agony.
Since someone mentioned Rumble Roses, I'd say my two favourite ryona subjects from that were Dixie Clemets and Benikage/Bloody Shadow. To me, they had the sexiest moans when placed in submission holds (or they did in the first game; I haven't played the 360 one much, so I can't speak for it much), and I especially love the way Benikage yells "NEVER!" as she starts getting folded up into a Boston Crab or whatever.
Others...I always preferred Gina over Lola in Beat Down on PS2. Maybe it's because she reminds me of Nina. (albeit a drug-addicted Nina who's had trouble sleeping for a few months) Oh, and how did I forget the Def Jam games? Fight For NY had some good chicks, but the move away from proper wrestling and into street brawling meant less submission holds, and a whole bunch of background noise drowning out the yelps. So from the first game, Vendetta, I'd say T'ai Monique and Penny were my favourites, with T'ai usually winning. She had more of a dominatrix-y look to her.
Oh yeah, and nobody's mentioned those Wrestle Angel Survivor games, either. I know they're just card games with static artwork, but...well, the voices are nice! And if we're counting them, I'd say Freia Kagami and Natassja Han were my favourites.