It really just depends on what aspect of spy you are going to be, say for the infiltrator part there is no best attribute all of them have no effect on it but going down the curthroat side of spy is different.
Knifer - simply put it's a ranged attack meaning your going to need RCT (Ranged Combat Tactics) for that you need focus also focus is good for the VD (Viral Deflection) to stop Viruses from hitting you and for force multiplier if you ever wat to start fighting in interlock thats the best hacker tree to use.
Assasin - These use melee attacks so your going to need MCT (Melee Combat Tactics) so you need perception. Perception gives you a better abilty to detect other people when they are sneaking or invisible and it is also very good for martial arts
But if your going into the hacker side of the matrix I'd go for reason because it gives more VT (Viral transmission)