Yoko Shimomura. As far as quantity, she is not too far behind Uematsu. according to Wiki, Uematsu has 47 video game compositions. Shimomura has 40. Her range of styles is also fantastic. It's kind of a shame that you only included her work with Kingdom Hearts in your poll--although it is great.

Shimomura's other notable VG works within the Square-Enix fold:
Legend of Mana
Super Mario RPG
Parasite Eve

Outside of Square-Enix:
Street Fighter 2 (!)
Mario & Luigi RPG Series (all 3)
Breath of Fire

What puts her above the rest for me is her range. I mean, you listen to the music in Street Fighter 2, then listen to the music in Kingdom Hearts...you probably wouldn't even think they came from the same composer unless you knew. (She also shares the first name of my overall 'favourite composer'... Wolf Kanno knows what I'm talkin about.)

Some of the other guys, Hitoshi Sakimoto in particular, you can listen to any game they do and be able to know the composer. You can almost take music out of Final Fantasy 12 and put it into Luminous Arc, or put Valkyria Chronicles into Tactics. Muramasa manages to avoid this by having a heavy Japanese style, but there are times when you can strongly feel Sakimoto's sound. Motoi Sakuraba falls victim to this too (he 'counts' as a Squenix composer--all of tri-Ace's games). I can't say it's a serious criticism, because good music is good music, but having a range of styles really bumps up my respect.

Actually, I encourage you all to check out an interview that 1UP just put online this weekend with her.
Yoko Shimomura (Kingdom Hearts, Parasite Eve Composer) Interview