The strongest engulfing candle is one whose body engulfs the entire range (including wicks) of multiple prior candles, forms at a key support or resistance level after a clear trend, and prints on above-average volume. The size matters: the more candles it engulfs, the better. The closer the price closes to the high (for bullish) or low (for bearish), the more momentum, the better. You do not want to see large wicks on your engulfing candlestick.
An engulfing candle on above-average volume of 1.25x or more of the 20-day average confirms that real capital is driving the reversal.