I bought veridian just because many artist seems to have it in their palettes, maybe for glazing, but I found it useless. The only green in my palette is hooker green, which makes sap green a waste of money as well.
Mine is Cadmium Red. My most-used is Alizarin Crimson (and/or other cool red). I just don't know what to do with Cadmium Red. I know it's a basic paint, but whenever I try to mix it with something, it ruins everything .
You just have to be careful... and you have to know how strong will be layer you are doing. Takes a time, but if you are not dare enough, try to practice first - execise on mixing - actually I never did it, but painters are two different types - first type needs careful plan, and then works, second canīt be bothered with planning and just take paints and paints. That is my case. Maybe you are the first type and need some practice before.
Cadmium red can be a tricky color to mix sometimes. It is considered an opaque watercolor, so it can mask out other colors. It is also a red that has more orange in it, so if you mix it with a blue, it'll turn to a dull purple or even mud.
Have you ever mixed cadmium red with yellows, like cadmium yellow? It makes super vibrant and rich oranges.