How to choose the best ADC clock?
As per the datasheet, the ADC must be used with a clock between 50 and 200kHz in order to get accurate results. Too fast and the S+H circuitry won't fully charge. Too slow and the S+H circuitry will discharge before the conversion is complete.

You must choose a prescaler that reduces the ADC clock from the clock control unit to this range. For instance, with a 8MHz system clock a factor of 64 will reduce the clock to 125kHz.