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.
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)
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.
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."
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.
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.
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.
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 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.