Assuming I'm choosing from cameras I already own, it would be my Yashicamat 124G - although it would be nicer if the screen was brighter.
The Mamiya RZ67 is tempting but there may come a time in life when I'm still fit enough to take photos but not fit enough to carrry the RZ around.
The Fuji GA645zi gets a lot of use now, but I know that the electrics won't last forever.