For me it was the
6510 - the little phone with the high-res screen with blue backlighting. This was at a time when everything else had crappy green backlighting, the blue simply stood out. Also had all the features of the 6310i, bar bluetooth. Was simply awesome back in the day and I found it amazing that they managed to cram so much into something that small.
One thing that most people seem to agree on - the greatest Nokia's seem to be the 6000-series
