A totally unscientific tally from the thread so far, here are the top ones:

Rolleiflex/Rolleicord 22
Hasselblad 500-series 19
Yashicamat 9
Bronica (various) 9
Mamiya TLR 7
Fuji 690/670 rangefinder 6
Mamiya RB67 6
Mamiya Press 5
Linhof Technika 5
Zeiss Super Ikonta 5
Rollei SL66 5
Plaubel Makina 4
Diana 4
Pentax 67 4
Koni 4