Freckles are sun drops sent from heaven, but they sure make it hard to choose a foundation
Personally, I just love freckles, I always have. My sister has them, and I don't, and it's always something I've been so jealous of. The grass is always greener on the other side, though. Sometimes, the issue with choosing a foundation when you have freckles is that a pale colour, to match your neck, may wash you out, but a darker colour, to match your freckles, can leave you with a line.
My gut reaction is to tell you to embrace them. Use a BB cream, or a sheer foundation to give the skin a bit of a glow without hiding them. But, if you're anything like my sister, you're not going to want to do this.
If you want to cover your lovely freckles, the way to do it is to choose a foundation to match your neck. If you have freckles on your neck also, you are going to have to squint your eyes slightly. The colour you see when your freckles and your skin blend together is the colour you are matching. The rule of thumb is that you are going one shade darker than your skin tone but slightly lighter than your freckles.
I would start by using a strobe cream as your primer as the light reflection in that product will create the illusion of a more even surface. A medium but buildable coverage liquid foundation is best for this, and a tinted powder over the top to set. Make sure to choose a warm tone foundation, as freckles are warm in tone; if you use a cool tone over them in a full coverage foundation, it can make your freckles look grey in colour.
If you are covering your skin completely, you will need to contour it to bring the colour tone and definition up a little. Like with any full coverage foundation, contouring will help give more shape to the face. Also, adding a bit of colour to the cheeks will help bring the colour back into the skin when you use a full coverage foundation.