"All of a sudden" is the correct phrase. "All the sudden" is a butchered idiom, and is probably based upon mis-hearing the phrase without ever having seen it in print. "All of a sudden" means "all at once". It implies stealth, or an unexpected or unpredictable action. "All the sudden" and "all of the sudden" are essentially meaningless. Go to any good online dictionary, such as dictionary.com or m-w.com and look up "all the sudden". It won't be listed. "All of a sudden" will.