Best to keep the complete A/D accuracy thing in proper perspective before you take too much effort and time looking for perfect components.
The Atmel A/D accuracy specification from the datasheet is:
• ± 2 LSB Absolute Accuracy
That's a pretty loose spec.
Your default reference voltage used in the A/D conversion is either the USB +5vdc or the on-board +5vdc regulator if using external power connector, neither which will be exactly +5.000000 and neither will they be the same.
The Arduino analog input pins are very useful for lots of things, but making high precision measurements is not one of them. External A/D chips with better resolution, internal voltage reference, switchable gain values all lend themselves to much better measurements if your application requires it.
Lefty