I have been several times on the weekends, I personally find Sundays to be the best, they are staffed for the weekend, but to me it seams like people use it as a travel day or something, so it seams less crowded to me. The last several times I have been, mondays have been way worse as far as lines etc. At least that has been my perception
If you can be one of the first in the park in the morning, I would pick Sunday over Friday. Sunday mornings are my very favorite time in DLR. It will get busier as the day goes on, but I think the crowd level would be comparable to what you would see on Friday.