Viper hands down.
Not for gameplay but because of personality. Amongst the legions of edgelords/ladies she has a bit more colour to her.
I just love it when you try to defuse the spike that's close to exploding and Viper just says what we all would think in that situation: "Shit, shit, shit"
Gameplay wise I go with Omen and Cypher. Lots of control and outlay potential. If only I could mute Omen's cringey voicelines...