Since you're looking for streets and neighborhoods, I would say you need to drive or walk through Downtown Littleton where everything is lit. It's very pretty.
We walked along 16th Street Mall last year and it was really nice to see something other than the Civic Center. It was really kind of neat. Not all of it is lit, but it's beautiful around the skating rink.
This one is really cool:
www.HighlandsRanchChristmas.com, view at 3175 W White Oak Lane,
Highlands Ranch. Here they time their lights to music. It really is spectacular! Not only do they put on a great show, but they also collect food (non-perishable) and toy donations.
If you drive into Christensen Lane Estates (Take Platte Canyon south of Bowles. Turn west on Christensen (across from Columbine Country Club). Take a right at the stop sign onto Christensen Circle). Many of the 33 homes here have holiday lights, but be careful driving as many people walk here.
If you're in the Littleton area, you should try this driving tour:
http://voices.yahoo.com/holiday-lights-driving-tour-south-metro-denver-10564253.html I've heard wonderful things about it! We tried to do it last year, but forgot the directions so we didn't get far, but it's something that I want to try again.
Also, The Denver Post has an interactive map of individual homes that have large light displays:
http://neighbors.denverpost.com/holidaylights.php
