As Linda and Dizzy posted, there is tradeoff in "best times to go" at DLR, because low crowds often coincide with minimum entertainment and ride refurbs. See "HydroGuy's High Season/Off Season Tradeoff Rule"
www.disboards.com/showthread.php?t=1199852
Everyone is different, so you can make your own call here. If you want to go during off-season, you will have to skip a lot of the entertainment options at DL and DCA, or schedule your visit over a Fri/Sat/Sun to see "weekend only" entertainment. However, with all of the AP holders at DLR (650,000) the weekends look like high season almost year round with so many locals visiting. So you will not have low crowds on weekends in general.
The better off-season weather will happen in April, May, Sept, and Oct. In the Nov-March time frame the weather may be great - or it may not.
It is a hard choice to make regarding visiting time. We like to go on summer weekdays, and the crowds have never seemed terribly bad to me. But it is more crowded than off-season.
Also be advised that since the 50th anniversary celebration started at DLR two years ago (and ended last year) attendance at DLR has been way up. Many folks have commented that off-season at DLR does not exist anymore. While some people have found this to not be entirely true, many others have gone to DLR during what they thought was off-season, only to be confronted with much larger crowds that they were expecting. To make matters worse, the shorter park hours during off-season is a double whammy when the parks are still crowded. Finding a truly reliable off-season at DLR has not been easy over the last couple years and most probably for the foreseeable future.