Since they nerfed armor lock, I'd have to say Promethean vision because you can see through walls. It's like the mod from CoD that lets you see through walls. Also, armor lock is cannon to the novels while the vision was just introduced for multiplayer.
http://m.youtube.com/watch?v=zGJEBiI85hM Revenge is a dish best served cold, and thank god; that creeper got what was coming for him.
Without a doubt armour lock, invincibility for several seconds that was unavoidable last minute if trying to splatter, beatdown, stick or use a power weapon. Compared to promethean vision that sends out a Shockwave on radar giving your position away, if radar is disabled Pv still has that very distinctive sound which is easily avoidable. Both are cheap and noobish though.
personally, i think jet pack is 10x worse than either. armor lock and promethean vision just slows down gameplay. jet pack breaks map design and many times renders stairs and lifts useless. it makes all map makers directly cater to jet pack users to make sure they dont make jet pack exclusive camping spots and just generally have to make maps bigger to further accommodate for that extra dimension.
on topic, promethean vision. what, dont know how to use radar to know where your enemies are? lets give players the ability to see enemies behind cover from close and long range. combine PV with everyone having sprint makes anyone using it basically a scared wimp. at least armor lock was patched so that you can at least kick people out of it and still has some usage, plus you dont have customized loadouts to make it even more powerful.
I honestly don't have an issue with Promethean Vision. Sure, it lets you see through walls, but it is really more like an advanced radar than anything else. Armor Lock, on the other hand, was basically a pause button for the user. I hated Armor Lock.