My favourite is because of lack of choice here in South Africa. First it is Com-art and then Daler-Rowney FW. I can't wait for our local Schmincke dealer to start stocking Aero Color.
I'm sure when you start using Schmincke aero color you won't be dissapointed, as Mika said it is ideal on paper, but Schmincke also have an additive which converts it to fabric paint which makes it better if you want paint on canvas or even t shirts.
When I bought my first brush I knew nothing about what paint to use, so I started with acrylic artists paint from my local art store, which is more of a gel than a liquid, I noticed quickly that I had to thin it every 5 minutes and it clogged too often, so that was a stupid choice, I did a little research and came across Schmincke and some others, I chose Schmincke only because I could order online and have it the very next day, I was astounded with the difference and now I see no reason to change.
I would like to try automotive paints, but this impossible where I live, in a fifth floor flat.
I also discovered that Schmincke provide a sort of felt tip pen or marker with varying size tips for use with aero color, totally refillable, I've ordered a set of all sizes, because I can draw better than I can paint, I think this will make airbrushing easier for me.