best sensor for determining which way is down?

accelerometers, magnometers, gyros each seems too offer possiblities

I want to add orentation awareness to my bipetal robot, so i need a sensor, that can tell me whether its on its side, back or even upside down?

which would be most native to this operation?

Obvious answer

Paul

that one is soo obvious its painful would be alot less code, too

Of course, the answer is an accelerometer. But the entire answer is somewhat more involved. The question to ask is "how do I know whether I am on my side, back or even upside down?"

If you are rolling around, the question is somewhat moot since whichever way is "up" at present may not be true in a few seconds time, so a gyro is useful to determine that. You may need to wait until you stop moving to figure it out. A magnetometer is not particularly useful to determine vertical orientation except over the (magnetic) poles, but compass directions can be a useful reference, including indoors (taking magnetic field distortions into consideration).

So in short in a bipedal robot, all three devices are simply essential, and conveniently available in "9 DOF" and "10 DOF" (including atmospheric pressure) modules.

The BNO055 absolute orientation sensor is a complete, self contained solution.