I like Musas. They are the most responsive I have found, and they have a very active forum where you can see what the girls are like and ask questions.
http://www.scortads.com/
For threesomes I'd recommend Angie and her girlfriend Demi. Angie is a tall curvy blonde with big natural tits and perfect English, and Demi is her real-life girlfriend so you will get real interaction. If you don't care about lesbian interaction, Lisbeth and Skarleth are real life sisters, so that's a fantasy fulfilled.
There are lots of other agencies. Popular ones include
http://www.cumintj.com
http://www.mexicolindobar.com
http://queenstj.com
http://foxystj.com/