For samatha, there is absolutely no substitute for sitting meditation. The other forms of practicing it are simply less than ideal alternatives for when you are unable to sit whether from physical exhaustion, pain, or for some other reason.
Just think about the body-mind relationship in each form of practice. In walking meditation, while the body is upright and invigorated, the mind has to engage itself with things like balance, physical obstacles, and the complexity of moving the body forward. Consequently, less mental power is available to direct toward concentration. A moving body is also a moving mind. Deep absorption requires a stable, single point of concentration. You simply can't cultivate that kind of attention when walking. Lastly - and this is important - there is an obvious limit to how far the mind can settle. The motion of the body stirs it up.
Conversely, when we lie down, we are actually too relaxed. A reclining body encourages a reclining mind. It is entirely too easy to fall into sloth and torpor in this position. Personally, I would actually say that lying down is less advantageous to samatha than walking meditation but others may have a different experience.
Sitting meditation - specifically sitting upright on a cushion...not with your back against something - is really the ideal. Your body is stable, stationary, and somewhat relaxed. At the same time, your sitting position encourages your mind to be upright and invigorated. The mind is likewise stable and doesn't have to involve itself in much beyond your object of concentration. Attention can also be applied to a single, unmoving point of concentration. Finally, the mind can also settle almost limitlessly.
As per qi gong - and this really should be a separate question - just apply the same standards! Qi gong is entirely too complicated to allow the mind to settle in the ways necessary to access jhana. While concentration is required for the effective movement of qi, you simply can't become fully absorbed in something that is moving around. Likewise, any movement of the body disturbs concentration in very obvious ways (in fact, that is one of the main reasons why it's best not stir during seated meditation). While it is possible to practice qi gong in a still, standing, sitting, or lying position, many forms, at the very least, require the arms to be held in various upright positions. All of these reasons make it less effective than sitting meditation.
However. If you want to practice qi gong before seated meditation, I say go for it. It will help you hit the ground running. Well, hit the cushion concentrating might be a better way of putting it. ;-)