With a striker fired SA, which would include your G21, it's important to use a non-deforming trigger blocking holster. Kydex holsters work well for this as do STIFF leather ones. Avoid soft leather and most fabric holsters.

The safest but fast holster IMO for your G21 is one of the Safariland ALS models. They come in both OWB as well as IWB versions. The ALS system is fast and snatch resistant while retaining the pistol well.