This.

And when we pack a customer bike for shipment, I always remove the derailleur. Easy thing, one bolt, usually bolts right back on without any need for adustment.

But if it was my own bike, I might be tempted to leave it on. I ride a small frame and know what I'm doing when packing a bike, so nothing on top which would press down in that area + space among the wheels usually means there is nothing which would press on it to damage it. With larger frames, you very often don't have a choice -- derailleur needs to come off just to fit it in the case.

Many, many new bikes are shipped with the derailleur attached, in a cardboard box...