Which type of cancer is the most curable?

Early stage. All cancers are curable if caught early enough to be removed or otherwise treated. The more dangerous cancers are the ones which spread early and/or are difficult to detect until it is too late.
The One Prevented. The best way to cure a cancer is to identify who is at highest risk and take measures to prevent that cancer from developing. Prevention includes avoiding known carcinogens: our greatest cancer foe remains tobacco. Short of this goal, the next best strategy is to "screen" for cancer: widespread testing of at-risk populations before symptoms develop (mammograms, colonoscopy, prostate, etc).