If you were to have only 1 Kelly and nothing else, then I agree with Cinderlala 28 Black Box with GHW is the most classic. But looking at it in context, do you have any Birkins or other Hermes bags? If you already have say a 30cm black box Birkin, then you can try something different for the Kelly. Also if you want a Kelly as a daily casual wearer, you should go for a more scratch-resistant leather and in retourne.
i thought about this myself for a long time.....and i finally found THE one. vintage (1959) 32 Black Sellier GHW. canvas strap. i feel like i can wear her anywear, day or night, dressy or casual. the age of the bag has relaxed the box leather enough so that - for a Sellier - she "feels" more relaxed, even though she is practically pristine. for me a 28 is too small and a 35 or 40 too big. black box GHW because its timeless and the canvas strap because it makes a Kelly wearable for everyday life.