When Marty McFly travels into the 'future' to 2015 we get to see the civic badge of Hill Valley on a police car. It features the clock tower on a disc, on top of an eight pointed golden star. The star itself is on top of several black and white warp rings. Finally a compass in the 'crest' position.
Marty coming out of the underground Court House Mall
Also in Back to the Future Part II we see the fancy logo of the 'Court House Mall.' It features a stylized clock with time hashes at 12:00, 3:00, 6:00 and 9:00. At the 1:00 position in a thunderbolt, obviously dedicated to when the clock was struck by lightning back in 1955.
Hill Valley City Seal
Here is a home-brew version of the town seal for Hill Valley, California. It is based upon the design shown in Back to the Future - Part II 1989.
Link to Part I
Back to the Future Flags - Link to Part V