M6.

Not the TTL but the "classic". Smaller than the TTL and the shutter dial works like a Leica shutter dial has alway traditionally worked. More recent model, less likely to need timely CLA. Has a meter. Leica lenses, especially the older Leitz models in moderate maximum apertures, are significantly smaller than all SLR lenses, including Nikkor/Nikon lenses.

Almost everyday of my life I carry a small Billingham bag with two M6's, four lenses, handheld meter, accessories, filters and 20 or so rolls of film along with all kinds of other non-photo crap. You will be surprised and amazed at how small these cameras are in comparison to most 35mm SLRs.

On a budget? Go for an M4-P. It's essentially the M6 minus the meter. Buy Voigtlander lenses--they're almost as good as (actually some are reportedly better) the Leitz/Leica lenses and cost a fraction.