Well civilized itself is a relative term, there were acts which the Romans and Greeks did which were abhorrent to Celtic peoples just as there were acts which the Iranian empires engaged in which were deplorable to the Greeks.
In simplistic terms, as in which faction would be most like the Mediterranean factions in terms of social structure and complexity which is visible in the archaeological record then it is a tie between the Getae, Aedui, Arverni, Boii and Arevaci. I know that's not the clear answer you are looking for, but I am afraid it is a far from simple question.
If I was forced to pick one I would say the Getae.
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