Most Helpful Guy
I will tell you right now that this question is way too open ended for me to answer well but I'll give it a shot.
Obviously being physically attractive makes a girl attractive. For me looks wise face is more important than her body but having both can't hurt. A girl who carries herself well with good posture and a good level of confidence is important. Things like well kept smooth shiny hair, nice smooth skin, and natural looking makeup can make any girl look more attractive so there are things that anybody can do to improve their look.
Personality wise it is very hard to describe what is desirable. You obviously want to relate to her and be best friends. The type of girl where you could spend a whole day with her and not even realize time is going by. Having a great sense of humor and being able to respect each other and understand each other. Its harder than I though it would be to put in words, but you'd know it if you found it.