IMO May. Warm weather but not too warm, low chance of rain (something many recent visitors wished they had put more though into) and still off-season so prices are not as high. And most major refurbs (think Splash Mtn) are complete in preparation for summer.
HOWEVER...
If you have never been to DLR before be aware that off-season works differently than WDW. Watch this forum and there are one or two posts a month from someone who was "devasted" or "had their trip ruined" because they misunderstood what off-season means at DLR and it did not meet their misguided expectations. Those who go with proper expectations have a much better experience and many claim it is the best time to visit.
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