This is a weird thing to try and answer.
I really want to say 35mm, and indeed, I do use the 35mm more than any other focal length (currently running 21, 28, 35, & 50).
However, I find that most of what I consider my best shots were taken with a 50mm.
But, I have thought about why this is, and I think I know why this happens (and this may be overly analytical).
You see, because I favor the wide end (over the tele), I'm used to working close, and as I'm sure many of you know, it takes a while to get accustomed to getting your a$$ in there and taking the picture--if i'm comfortable, the subject is comfortable, hence the closer I can get, and so on. Being invisible from 3 feet away takes a helluva a lot of practice, and I still need work.
Anyway, you do this for a while (or rather, I did)--cut your teeth on the 28 and 21--and then the 35 is really comfortable, but the 50---shooting with the 50 starts feeling seamless. I feel like I don't even have to get involved when I shoot with the 50 now, no matter how close I am, and that level of comfort and confidence, I think, transfers to the subject.