Cloaking a bunch of reavers and marching them straight into the enemy base (1v1).
That became pretty difficult to pull off after the conjurer nerf, because if you use it with its cloak, it is incredibly slow and has a huge energy upkeep. However, a couple of times I managed to be successful with an alternative where you use a strike commander and its radar jammer instead of that conjurer. True, you are not cloaked, but the enemy can't see the dots approaching and that is usually enough. You need a bit of luck to not get revealed by a scouting glaive. Maces can be used instead of reavers.