I know of three great spots to see amazing Christmas light displays. One in in Chino, off of Riverside Drive. Another is in Rancho Cucamonga, due north on Benson, and the last one I know about is in Pasadena, North of Foothill above the 210 freeway.
The Chino lights are on Lemon Place and Fillmore Street (near the intersection of Walnut and San Antonio). I grew up on the next block and remember how the lights started getting more and more outrageous every year. We had to put more lights every year just to keep from looking so bad. The story (the way I vaguely remember) is that one person on the corner put on the roof lights that said something like "Yamaha Rules." His buddy down the street then answered with "Suzuki #1" (or something to that effect). Then the first guy responded with a bigger display. And every year they tried to top each other. The neighbors then just putting up more lights to try to keep up. Eventually people started to see the light off the 60 fwy and came over to see the show. The Chino police had to start getting involved to control traffic. Years after the originators moved, the local neighbors have kept up the tradition, but not as over-the-top as it used to be.
The two I know of are already mentioned. Lemon in Chino used to be our tradition, but I haven't been in probably 20 years so I don't know how it is today. It was really awesome when I was a kid because a whole neighborhood goes all out.
The other one is a couple of blocks from my Mom's house, off of Sapphire and Thoroughbred in Rancho. Similar thing, where the whole neighborhood goes all out.
Both get a bit trafficky, but nice to take the kids to if you have patience. We're crossing our fingers that Thoroughbred will still be going tomorrow as we're headed that way. My kids have never been.