It's all really dependent on what you shoot and what your price range is.
but the nifty fifty (50 1.8) is an excellent lens for the price, and coupled with a reversing ring, makes a nice macro. Gives you a chance to try shooting in darker situations, portraits, marcro, etc. etc. Warning though-fixed glass is addictive, and expensive
Now do you have to buy a lens, or are you open to other options? A nice Bogen-Manfrotto tripod, a speedlight/strobe, a battery grip (OEM or not), etc etc.