I am very partial to the GSP Ultra Nylons, (high tension), the Marco Pereira model. They can be ordered over the web at
www.gspguitar.com. Besides the tone, I also like the fact they seem to stretch out and stabilize sooner than other nylons I've tried. Then they last a loooooooooooong time.
One thing not to try... Martin classical ball end strings. Terrible stuff. I think all the price went into the ball on the end and nothing into the quality of the strings. I gave them 3 days and they were gone.