I used to have a love affair with Delta 400. Had been bulk loading and shooting it non-stop for 3 years.
My favorite general purpose developers for Delta 400, sorted by awesomeness:
And the following two will give you a crisp & sharp image with well-defined but also tightly packed grain. Again: the grain is actually fine, but it's well-defined and therefore more visible. Sometimes I crave this look vs Xtol smoothness, it's a personal preference thing
Developers to avoid:
Delta 400's grain loses its charm with both. It starts to look rough and irregular. The weirdest thing about Delta 400 is how its grain looks completely different in different developers. In fact, the grain character changes quickly with exposure too. But if you don't scan at high resolution or don't wet print large this shouldn't be a concern.