It will be very difficult to answer that without knowing what you're designing.
You have several options...
Start with a plane which allows the part to be created in an as-assembled orientation
Start with a plane which allows the part to be created as if it were sitting on the desk in front of you
Start with a plane which orients the part to best suit the manufacturing process
Whichever you choose, try to create the part with a logical origin point, and use symmetry wherever possible.
While there is no right answer in my opinion as long as you stay consistent through the whole design.
Now for my other two cents. I generally use the top plane for the floor. The front view for when the product is coming towards me and right for a side view of the equipment. So if I was designing a car: top would be road, front, I would be looking at the grill and right I would be looking at the driver side (third angle). But follow my first statement for your own needs.
I'd like to reiterate what Kelvin said about symmetry. Much of the work I do involved symmetric models, so the right plane almost always bisects the model at the centerline (so half the model is on the right side of the right plane, and the other half is on the left). That makes it much easier for mirroring and mating.