Such a difficult question,
For Hunters, Jumpercables, just last night we lost our OOC Rezer, hunter used FD and rezed a priest, poor Sulfurion, I bet he thought he had us beat.
For clothies, well one of the helms, ya the MC hat is nice, but the rocket helm is my fave, and time will tell which is the best,as I see situaions for both. Side note i have been playing arround with a lock, and am a demonolgist, will grabbing a "Pet" with the mc helm nullify the bonuse of demonic sac? anybody? I dont think so, and as a result, it will be my fave beofre long, but the rocket helm is just plain funny.
Warriors, all about the force reflective Disk(probly cool for shaman/pallies too, but I only got one on my warrior), and stuff like the flame reflector(amusing how most mages cant take the dammage they deal)
anybody with a nice pile of HP, Sapper charge, Heck pertend your a warlock for a moment, and burst in to flames, How fun. Some other favorites, NetGun, Death ray/Dragon gun(this only works well vs huge numbers of oppents)
But for me it is no single item, it is all of them togather that make the profession exelent, If I had to pick one thing,Explosives, namely granades, Ranged AoE, with a stunn, that can be "Cast" on the run, ya thats the winner for me.