Which one of them qualifies as all purpose flour?
If this all purpose flour has got some special name, please enlighten me.
Of those two, Maida flour is the closest to 'Western' all-purpose flour (which is an American term. It is called plain flour in Britain). Atta flour is made from durum wheat, which is used for things like pasta (and of course Indian breads like chapatis).
Maida flour is made with 'regular' wheat, but it also has a lower protein content than all-purpose flour (8-10% as opposed to 9-12%). The impact of this lower protein level will depend on what you're cooking. Maida flour should be good for making cakes, for example.
All purpose flour (sometimes abbreviated to just AP) has a protein level in the middle of the range for wheat flour. Flour with high protein level is known as 'bread flour', and flour with low protein level is 'cake flour'. Flour containing more protein will develop more gluten, making for a stronger, stretchier, chewier dough. That's great for bread, particularly breads that you want to have a chewier texture and firmer crust such as baguettes, sourdough, rustic breads, and so on. Lower protein flour is used for softer products such as cakes, muffins, and biscuits (in the American sense, meaning chemically leavened dinner rolls, not the British sense of cracker or cookie). With its medium protein level, AP can be used for either breads or cakes and will give a product that's neither very chewy nor very soft.