I'd like to hear a collection of schematic reference designator lettering preferences.

Let's include the whole spectrum.

aka J

Here's what I use:
- - - - - - - - - - - -
U - Integrated Circuits
R - Resistors
C - Capacitors
L - Inductors
D - Diodes
Z - Zenor Diodes
Q - Transistors
B - Batteries
X - Crystals
F - Fuses
K - Relays & Solenoids
SW - Switches
S - Speakers & Buzzers
J - Male Connectors or jumpers
P - Female Connectors or pads.
N - *Display devices full spectrum
M - Mounting related hardware
- - - - - - - - - - - -