I get a blood pressure reading of 147/93 sitting and 125/76 lying down. which one is the correct one to use to diagnose hypertension?

See a doctor: Those values do not make sense. If anything your BP should be higher when you lie down. Have a doctor check it.

Sitting: Although sitting, standing and supine blood pressures are all informative, sitting BP is used as the official BP by most physicians. You have mild hypertension if your average BP sitting is 147/93. If on BP medication, be sure to monitor your standing BP as it may fall too low on some medications more than others.

More measurements: To answer correctly, it is better to get more measurements, in one position. Sitting up is usually the measurement that people use in determining blood pressure. See your primary care provider for more information.

Sitting: It also presumes that you were in a stable condition, and had not just exerted yourself. See your physician for an opinion.