1) Palm Strike (unconditional leading off-hand, not an attack so it never misses and low recharge)
2) Twisting Fangs (the only way to deal a deep wound, which every assassin should be able to give)
3) Mark of Instability (it knocks down; it's an hex, which leads to...)
4) Golden Phoenix Strike (my energy management skill and a conditional leading off hand)
5) Critical Strike (very nice damage at 15 CS and it gives back 8 energy with zealous knifes, knocks down thanks to MoI)
6) Falling Spider (self explanatory)
Sorry for not choosing just 3 skills but it wouldn't be possible, an assassin works so well thanks to chains, not single skills, and I found out this chain works extremely well for me.
