So the new Assault Vest item added in X and Y adds an additional 50% to the Special Defence stat at the cost of being perma-taunted.
Obviously similar to Eviolite, but it doesn't effect Defence but can be applied more usefully to many more pokemon. The clear choice of this item is to put it on something with a reasonalby good Special Defence stat to being with. I want to explore multiple uses for this item. A few ideas I've had below.
Sp. Def oriented sweeper as a counter to beefy special attackers (Good choices here might be Gyrados and the Togekiss)
Already beefy defenders in need of additional Sp. Def, not so much used to stall, but outlast opponetns with moves or abilities that drain health (Giga Drain, Drain Punch, etc).
Scouts that can utilize moves like Fake Out and U-Turn.