The best opener as an operative from 1v1 is Sap > Volatile Substance > Apply x2 Hots to yourself > Snipe > Stun/Backstab/Crippling Slice/Corrosive Dart.
This opener allows you to use 5 GCDs before sap breaks with the last move in the rotation depending on the situation. For example, I would use corrosive dart after snipe if I knew that the damage generated from snipe and corrosive dart activating my volatile would guarantee kill my target .The dot generated from backstab has a slight delay before it starts ticking while corrosive darts has no delay and will activate the volatile substance detonation immediately. It would be very rare for you to come to that situation of having to use backstab over volatile but it does exist.
For those that are confused about the snipe used.... Using snipe after flashbang or sap is just added damage since casting counts as your GCD. Sniping breaks the sap and allows you to immediately use another ability after it with no delay. Only good for 1v1 situations obviously. Too many things could happen in a group scenario trying to get this off.