Which form of applying Minoxidil is better? Injectable or topical?

This depends on the quality of topical minoxidil and its strengths but if formulated well, topical is the safest most effective for hair loss at 7-9% strength used 1-2x daily.  Injections can be done via mesotherapy along with progesterone and vitamins.  However this requires you to frequently visit your doctor weekly for the injections. Orally and intravenously minoxidil was used as an antihypertensive agent although its use for that now is less common in the US. 

There are two forms of minoxidil, one is the topical which comes in many strengths, the second is the pills which was originally used for the treatment of high blood pressure. The pill, which is rarely used for hair loss in the US and can bring down the blood pressure of the person taking it, is more effective at treating hair loss than the topical