Despite being an SLR user since the late 1960's I would use my M3, this quickly became my main 35mm camera within weeks of buyin it in the late 1980's.

If I ndidn't take a Leica I'd use a Pentax MX, perhaps and older Spotmatic F. There's no way i'd take a Nikon F, but an FM/FM2 woulod be a good choice in terms of weight & size.