What is the most uncommon key signature for rock music?

Generally, the most common keys for rock guitar bands are "E" and "A", two keys easily played on Guitars. Most of the songs on the first two Beatles albums are in the key of "E", for example.

For rock piano, "C" and "G" tend to be the easiest used.

It should be noted that the better bands can play in almost any key, but the keys of "B" and "F#" tend to be avoided.