MCU ADC and signal conditioning circuit layout
In general the RC filter should be as close as possible to the ADC pin.  Much of the layout issues relate to the other traces in the neighborhood.  Most of the noise (other than shot noise/etc) is coupled from somewhere.  Running traces at right angle to other traces with fast edges, using a 4 layer board and putting a solid ground under your ADC signal trace, and putting a resistor between the ADC input and ground (to lower the input impedance of the ADC and reduce noise sensitivity) will all help reduce the noise.