well, it's hard to pick out one, because I love all yarns, but when I think about it...
I'm not a really fast knitter, not only because I'm slow, because that's getting a lot better, but mostly because I don't have much time to knit. I usually make sweaters and that takes really long. That's why I voted bulky, because with that yarn, I can still finish a sweater quick (although I've only made 1 sweater with bulky, but I really love that one)
of course I don't use bulky for socks. For socks I prefer fingering weight.