My preference is SN, AN, MN, DN/KN
The reason is that generally speaking the suttas in the SN are the oldest, followed by AN, then MN. That's a very rough guide because obviously suttas were compiled at different times throughout all the nikayas but I think this is a good way to start if
you plan to read them all.
However, it must be acknowledged that reading them all does take a while, so either reading suttas online or a compilation such as Bhikkhu Bodhi's "In The Buddha's Words" might well be a better introduction.
Unlike TheDhamma, I think the Digha Nikaya is a terrible place to start! It's quite verbose, comparatively speaking.