This video compares speed and compression rates for winrar, 7zip, winzip and window's built-in zip utility. I used a mixed of photoshop/png images, and the program files folder to test. Run on Windows 7 64-bit.
Result:
7Zip had the best compression, but winrar was almost as good and much faster. Winzip fell behind in all areas. Built-in zip was fairly fast but didn't offer good compression.
See video for more details.