Well then, M6/35 and maybe T3 too, BW400CN.
Honestly, I am in Paris 3 days a week, and I don't think a sole 35mm would do it for me AT ALL.
a 35 for snap shot, may be. But if you start shooting ancient buildings, churches, small streets of the quartier latin or Marais, this may turn out quickly frustrating…
At the very least, a 28mm would be my choice.
And considering the awesome weather we are having now, I would choose nothing with higher sensivity than 100 asa.
Maybe the best thing would be a cam with 50 asa, and a another one with a higher sensivity film. Paris is an all black and white material town, and when the weather is really bright the contrast is quite rude.
I am not representative of anything, but here what I have in my bag when I am on duty in Paris :
Leica IIIc with elmar 50mm f/3,5 Neopan 1600 or Agfa Scala 200 @ 1600 (for inside shots)
Leica IIIg with heliar 50mm f/3.5 Adox CHS 25 or APX 25 ; but fuji acros is fine also (for ultra bright days like now)
Leica MP with CV 28mm f/3,5 with velvia 50
And I keep my summicron 35mm in ltm at reach.
And I can swap lenses if needed, of course. I use a small CV minifinder.
Unfortunately, I don't have a 21mm for my range finder, not yet.
All this hold in a (small) shoes box.
And if you want to process all your films in situ, not cheap but very reliable hand made development services can be obtained at Arka lab, rue notre dame des champs.