In the context of mathematics, I would say: "Negative five over two." Using 'over' explicitly states it is a fraction, and can be used for any x/y: "x over y". You can say "Negative" or "Minus", it really makes no difference. Personally I use "Negative" to explicitly state that the term is below zero. As a Physicist, I might need to read a lot of Maths, and this helps to remove confusion. "Minus" is used more around where I work as "x - y", "x minus y". If someone says "Minus" to me, I might assume there is some number they are subtracting it from!

Minus five halves. Minus five over two. However, given the denominator is something simple like two people might be more likely to read it as a number: 'minus two and a half.'