What's the most useful thing in the room you're in right now?

Depends on the situation.
Currently? 40 pound box of cake mix I'm probably going to launch at my store manager.
- Is it the 40 pound box of cake mix that kills? Or is the person hucking the 40 pound box of cake mix?

My cat. Everything else in my room brings me either misery or a feeling of meh. Cat brings joy, which is difficult for me to obtain.

The toilet. Lost count of the number of times this has saved me from getting dysentery or something and it's so convenient when it just pushes the shit away from my house and I never have to see or smell it again. I almost feel bad that I repay it for all its done by repeatedly shitting in it.
- I was going to say the paper but you're absolutely right

Me? Unless I’m excluded, then the laptop. Although the room itself is pretty useful, but I don’t think I consider it “in the room”.

A 5gal jug full of water. (I don't have AC and it's 32C/90F inside)
- Consume salt! Water on its own isn't great for hydrating when you're continously sweating, Gatorade exists for a reason (to supplement the nutrients lost to sweating, not just the water)
  
  No worries. Gone through 2.5kg(~9lbs) of Gatorade powder, just this summer. LiquidIV got way too expensive after the first box.

Probably my phone, and that probably applies to most rooms frankly if we're talking about potential things I could need to do

The answer is electricity. It's so useful, we take it for granted. Your phones, computers and laptops are not useful if you can't power them.
I'll give the people on the dunny their due: toilet paper is still useful in a room without electricity.
- Your point is a good one, though I'd like to point out that electricity is so useful that if I have power and water in the room (and a bit of prep time) I can make oxygen.

My wife.
- Same! Mine's smart, entertaining, manages my social life, probably contributes more to society than I ever will and on top of that, for reasons that still escape explanation, actually wants to have sex with me which is probably the only reason I never turned into an incel. She is easily the best thing in this house, myself included.
  
  Right there with you. She’s amazing. Plans all our trips. Organizes the kids. Lets me do the things I do best. Perfect travel buddy. To wives!🍻

That's a tough one because I'm currently surrounded by computers and tools. I'm within reach of my smart phone, one of my laptops, I'm typing this on my desktop PC, I'm within reach of three multitools, a power drill, and my away mission tool bag. So I think I'll give a cop out answer and go with "The several thousand liters of air filling the room."

My phone. It might be my laptop, but it's currently not working. If you consider humans, me.

Probably my hatchet, first aid kit, and a duffel bag I keep packed with all weather gear and a small tent as well as my headlamp, survival knife, and half a dozen MREs
I'm not even a "collapse is imminent" prepper kinda guy but it's better to be safe than sorry, you never know.

Laptop probably. Linux, tmux, i3, vim, GCC, GDB, Pdb, gplaces, links2, curl, wget, netcat, awk, sed... what more could be needed for happiness?
Idunno, but it's preloaded with shittons of tools I don't even know the use of yet (thanks kali). Excluding that probably my lensatic compass. It's nice, quite cheap and not magnetically dampened but it does the job.

The manager likely thinks it’s him, but it’s actually all the workers that do the work.

Air. Can’t go more than a minute or two without it, and there’s enough to share!

Most useful? Without a doubt, my laptop. The amount of things that can be done with a modern computer is pretty stunning, and a portable one is arguably more useful.
Other than that, maybe my car keys because the amount of things you can do with a car is stunning, but you need the keys to start it, and the car isn't in this room.

I'm outside so I'm not sure how to evaluate "the room". My phone is the most immediate at hand (pun intended) useful thing right now though.

gotta be either the tiny computer i'm typing this on, or the large, sturdy, wooden stick in the corner. if i had to pick to keep just one though, it'd be the phone. a sturdy stick has a million and one uses, but it's also fairly easily replaceable.

My pc but that's too easy so I choose my bed as the very next thing. Good comfortable sleep is important.

My field recorder! Chilling after band practice right now, and being able to clearly listen back to my jams has been invaluable in progressing my musicianship.