Get the 50mm F1.8...it's the best value because it's cheap, fast and sharp. If cost is not a problem, then get the F1.4...or what ever you want.
Yes, using flash will make it possible to get shots in some lower light situations. However, shooting with flash is a whole different style of photography than shooting without one. A flash will have a limited range, you can't light up a whole landscape with a regular flash shot.
A fast lens (F1.8 ) will also allow you to get a very thin DOF...which a slower lens just can't do...flash or not.
It really depends on what kind of shooting you like to do and your budget. Use your camera for a while with those lenses. If you wish you had more magnification, then get a longer lens. If you find that you are wishing for a wider view, then get a wider lens. If you are finding that you want to get closer than your lens will focus...then get a macro.