While I currently shoot several different emulsions - color (Ektachrome E100G) and black and white (PanF Plus, FP4Plus, HP5 Plus and Tri-X) - with my Leicas, I generally rely on Tri-X or HP5 Plus for most of my candid street shooting. Both Tri-X/HP5 are sharp, forgiving, have moderate grain and can be "pushed" if need be. The nominal ISO 400 rating allows you the advantage of higher shutter speeds (read: hand-holdable, with a rangefinder) for a good portion of the day (i.e. allows you to shoot sans flash!). I generally rate these two at EI 200 for most of my work, pushing to EI 1600 if I happen to be shooting into early evening.