Dividing by 1023 or 1024? The final verdict on analogRead

Not at all. I meant to ask what I asked. Perhaps more a more explicit question. What is the lowest voltage (in microvolts) that will produce an output of 1 count from the ADC?

P.S. Based on the project work last night, an Arduino UNO ADC outputs 0 count when the ADC input is connected to 0V (ground), I'm not sure how that can be incorrect.