Sigma 20 or 24mm f/1.8
I have the Sigma 20 f1.8, i really like mine a lot, with that said be careful before you buy one, if you want to shoot it wide open a lot. I won t say is bad or good. The lens has a unique character in this respect, you may like it or not. It s sharp in the center, but it has low contrast and coma, at medium distances it has a nervous bokeh, but when you get close to the subject and you can get very close, 20 cm from the sensor the bokeh improves. My needs a micro adjustment on my D700. In short it s a lens for the heart not for the brain, a reallly unique and most importantly a creative one. Some samples:
One good lens would be the Samyang 24/1.4, if you don t mind manual focusing.