Nearly impossible for me to pick just one. Therefore I will choose among categories:
subminiature:
My Minox B.
pocketable:
My Ricoh GR-1 with the Olympus XA as second choice.
instant:
My Polaroid SLR 690, although I prefer the tonal qualities of the Polaroid Time Zero film in my SX-70.
pinhole:
My Zero Image 2000 6x6; no frills, as basic as possible.
35mm rangefinder:
My Leica M3 (ss, st, pv, with dof indicator in patch) with 50/2 DR Summicron.
35mm SLR, non-electronic era:
My Pentax SP500 (without a battery since I don't like stop-down metering) with Super Takumar 55/2; this is the camera I'm used to since I used it exclusively for 15 years. In the future, I think I'll prefer the Nikon F or Exakta 500. I enjoy the Nikon F2, FM3a, FM, and Nikkormat FT2 as well, but the simpler cameras allow me to concentrate more on what I'm seeing.
35mm SLR, electronic era:
My Canon Rebel G (known internationally as the EOS 500N) with EF 50/1.8 II; lightweight, fast, easy to use and amazingly flexible. Although I enjoy my Nikon F4s, Minolta Maxxum 7, and Canon Elan 7NE, the simpler Rebel G is effortless to use.
Medium format rangefinder:
My Fuji GW690III.
Medium format SLR:
My Mamyia RB67 Pro S with 140mm Macro C. My Hasselblads are of high quality, but I treat them like jewelry.
Medium format TLR:
My Rolleiflex 3.5F.
Large format, technical/field:
My Pacemaker Speed Graphic 4X5.
Large format, monorail:
None -- looking for an 8x10.
...and yet, the one camera I think is the most lustworthy: the Minolta SRT-101 with 58/1.2 MC Rokkor