Most common leak overall - shrader valves.
Most common damage leak - hose rubs on the steering column
Most common installer error leak - hose melts on the crossover pipe
Other common leakers - O-rings at condenser, low pressure switch.
Not as common but we've seen a few:
Evap core (usually a fast leak)
Compressor bearing;
Return hose about every 5 inches for the whole length of the hose;
Either hose when some idiot replaces the gas tank cover screws with longer screws, or leaves out all the washers. Pokes nice 8 mm holes in the hoses where they pass over the ends of the screws.
Crossover hose due to the sharp bend in the hose, and the high pressure switch/blowoff valve.