I use 'Old Holland' oil paints. These have a very high percentage of pigments. This makes them ideal for wet-into-wet painting, as opaque paints are as opaque as they get. Paints behave as you expect, ie. paint in red somewhere, and the spot actually becomes red, and doesn't dissolve into the paint that is already there. Also, when mixing the paints, they behave more closely to what they should theoretically (ie. like in the color circle). As opposed to less pigmented paints, which get dull (desaturated) easily when mixed. The high concentration of pigments also allows me to use more medium when I want to.