I mean, the obvious answer is Crowder, because he will be back unless he gets a ridiculous offer. He's a restricted free agent, and until he signs a new contract, he'll count less than $1.2 million against the cap. So he can be resigned and not get in the way of any other plans offseason plans.
Datome is also relatively cheap and is also restricted, but he's more expensive than Crowder in terms of his QO and cap hold, and his cap hold is probably higher than he'll get in free agency unless he shows even more the next month or so. So he might come back, but I don't see him given a QO, and it won't be decided until after the Celtics look for other free agents.
Accordingly I'll pick Bass, because he's been here the longest and fills his role well. Jerebko is nice, but he's somewhat duplicative with KO. So I'll say Bass, for the right money (which is in the $4 million range).