Canon EF 50mm f/1.8 II Primarilly a good solid inexpensive fast lens. If you are working outdoors a lot this little guy will get you a lot of bang for the buck!
Canon EF 50mm f1.4 USM A Nice step up for not a huge investment. The faster lens if you look at it has a nice big chunk of glass in front of it. And that is really the whole point a bigger lens gathers more light and gives you sharper and brighter images, everytime.
Canon EF 50mm f/1.2 L USM HUGE Lens. Superfast and extreme light gathering (Did you know that at one time Canon made an F1.0 50mm?) but the big kicker here is the L series glass. You simply cannot beat Canons flourite glass with anything but a Leica

waiting for the Nikon screams
But Nikon has beautiful Lenses in comparable price ranges. Your best be is if you have a good camera store near you they should stock the 1.8 and 1.4, go and ask them to let you put the lenses on your body and snap a few shots there at the shop then take them home and look at them. Be sure to
print the best two one from each lens and then you'll really see the difference.
Raven