I agree with the last post, for the Canon 35/2. Primary problem is it's not common, and usually pricey.
The FSU 50/1.8's are normally an excellent lens. Same is true for the humble Canon 50/1.8. I prefer the chrome to the newer black, because I like the heft of the chrome lens. If you find a good one at the right price, a great choice if the Canon 50/1.5. As with the 35, it's not easy to find, and still pricey.
I usually carry a small kit, with a P and the 35/50 mentioned above. If I need a 2nd body, I'll add the 50/1.2, just in case I need the extra, and use it on a Canon 7s. I also have the 50/0.95, but not usually used except for theatre or night street work. Also, they are heavy, but do make effective, but expensive, weapons. if really needed. Actually cheap now, considering the hassle and expense of a decent H&K or such.
Harry