Holga 120 CFN
As much as I love my Mamiya cameras, I am absolutely in love with the Holga. This simple plastic box with a spring shutter was my first medium format camera and I still carry it with me everywhere I go. It doesn't render much control over the exposure, it rarely rolls the film on tightly, sometimes the back falls off if you don't put tape over it, and the negatives are usually over or under exposed. But yet they're cheap and light, and the images they make have a wonderful distinctive look and feel, different from what I get with my other cameras.
This hit or miss aspect of this type of photography keeps the intrigue in the act of photographing, breaking the expectation of predictability, and leaving room for experimentation and unanticipated surprises.