Cream.
99% of basses seem to have black pickups, so I always really enjoy seeing cream covers.
The problem is, they don't work on every bass, aesthetically, and of course, they are not always available, if you have pickups that don't have removable covers, for example.
Wood covers are nice, too!
http://farm2.static.flickr.com/1071/...1bfb9205_b.jpg