My Vote for most useless piece of X-Com equipment. . .
The Electro Flare
I never use it. I never even order it.
Whenever I want to make light, I set something on fire, preferably with some incendiary rounds.
Carrying around a item weighing 3 pounds, which only illuminates a 2 tile radius, must be thrown, and isnt going to help you see through smoke (or some critter which is flying). Not to mention I dont think they are recovered at the end of the mission, and an explosion can destroy them.
#ultimateFail
Night missions are hard and they are supposed to be. And a Flare does not really make them much easier to manage. Best way to attack aliens shooting from the darkness is to circle around on them so you can get close enough to see them. Either that or close enough to see them walk around or make noise (ie, not-hidden-movement) so you know their general vicinity and can pepper the area with grenades.
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Next useless item:
Auto-Cannon Incendiary ammunition
why go small when you can go big? Just use an Incendiary Rocket (I keep one spare if I really need it on a night mission).
Third tier useless item:
All Those Corpses you keep finding on the ground.
Sure its important for research, but after you've researched the corpse you have no further use for it except for selling it. So basically its just this game's version of "you found some gold coins".
Fourth tier useless item:
Motion Scanner
Indeed it can be useful, in the dark, or in a network of close quarters when you got scary creatures like cryssalids and mutons wiht heavy plasma guns running around.
But it only has a reach of 10 tiles at most so its not going to pierce the dark at any good distance - you will still get reaction fired on just as much, and even in the dark by the time you can get close enough to detect someone on the Scanner . . . you can just see them.
So . . . Very limited functionality, especially considering it costs research and manufacturing to get it.
Start of Useful, but very poor use items (as in not useless, but it sucks . . .)
Pistol and Pistol Clip
Its a weapon, it kills things, and its compact.
But it has no auto fire, and most of the time one shot wont do it. Its a death trap except to finish off something which has already been hit (and even then it might not do it).
But tons of people cry foul about that so thats why there's a mod.