Best is the one you read and use. Check public library to get started. Browse a bookstore (if your community still has bricks & mortar stores and they stock bike-related titles). Or buy from your local bike shop (they will appreciate your business!). Besides Qcwtom's good suggestions, Friel's "Cycling Past 50" is an abridged Training Bible for the rest of us (non-racers). On line/free: Bicycling Magazine has on line training plans for many types of events - you can just take one, or take ideas from them.

Abdon is right - without context (current fitness, health, your cycling-related goals), it is impossible to be more specific.