I use gouache, watercolour, acrylics and more recently oils. Gouache and water colour though, is what I have mainly used in almost all of my work over the years.
Gouache, in case you did not know, is an opaque water based paint containing gum arabic. It’s a bit like watercolour but gouache is a not transparent like watercolour – it is opaque as does not let light through to other painted layers. Depending upon what sort of effect you want, gouache can be applied straight from the tube, diluted and applied like watercolour or, as I do, mixed with water in varying quantities and used with watercolour. I find they both work well together if used in the order of gouache first, then watercolour… otherwise the gouache will obliterate any watercolour glazes. I’m only saying this through experience and do not want to give any lessons, but merely to explain how it works for me.
When choosing paint, I will always try and use the best quality artist’s paints available. This also applies for oil and acrylic paints.
I have found (for me) that mount board or illustration board provides an excellent smooth acid free surface upon which to apply the paint for my style of painting. I do use watercolour paper too, which tends to be the thicker paper, about 140lb upwards. Mount board is very thick, after all it is for mounting pictures, but I find it a superb surface to work on for what I do.