What's the highest precision achieved for an ADC?
Definition from Wiki: -


  Effective number of bits (ENOB) is a measure of the dynamic range of
  an analog-to-digital converter (ADC) and its associated circuitry. The
  resolution of an ADC is specified by the number of bits used to
  represent the analog value, in principle giving 2^N signal levels for
  an N-bit signal


Quote from Atmel: -


  In most cases 10-bit resolution is sufficient, but in some cases
  higher accuracy is desired. Special signal processing techniques can
  be used to improve the resolution of the measurement. By using a
  method called 'Oversampling and Decimation' higher resolution might be
  achieved, without using an external ADC.




Oversampling - take 4 consecutive samples and combine them to get one more bit of resolution; take a fairly standard 18 bit ADC and oversample by 256 to get a 22 bit ADC. Oversample by another 256 times to get a 26 bit ADC...

Do you see where this is going?

If noise is present and causes dithering of the signal, you can make any ADC have one extra bit by averaging/decimating 4 samples so, average as many as you like to get a higher resolution but clearly the price to pay is proportionally lower bandwidth and accuracy.


  What's the highest ENOB ever achieved?


What do you want it to be?



Footnote - a sigma delta ADC does exactly what I've described above except, it manages out of band noise much better and therefore gets a better yield on increased bits per converted samples averaged (or decimated).

It only uses a 1 bit ADC (a comparator) so clearly this technique works but it doesn't have to use a 1 bit ADC. It's all about noise filtering: -



The noise from a sigma delta ADC is progressively higher at higher frequencies due to the use of an integrator in the signal path - this forces noise to be low at low frequencies and, after decimation this yields a net benefit in resolution compared to just a conventional ADC that has been over-sampled and decimated.