Complicated.
Things like: how I feel on it (and does it make me go ARGH! in equal parts pain from forces and enjoyment), how I feel when I get off, will I ride it again, what lasting impression does it leave, how does it compare to what I have already riden and so on.
Nemesis is my highest rated ride because it is incredibly forcefull and makes me go ARGH and WOOOAH, gives a rush of blood to the head when I get off and leaves a huge mark on me.
Also, how much did it surprise me and match my expectations, and how does it 'feel'.
Most importantly, does it feel like it has a soul or does it feel like cold, hard, dead steel.