Have a few useful tools like screwdriver, knife and pliers. But also the thumb would totally also be a usb drive. Mostly for the non stop thumb drive jokes I could make.

Go go gadget fleshlight!

If there's still room after the flamethrower and various vibrate functions I guess I could fit a nail clipper or something

I misread this as aphrodisiac so I would like my arm to fuck

Scratch the part in my back that I cannot reach

Go go gadget arms!

Magnetic charger and phone holder in the forearm area. Prehensile fingers with lights on the tips and one with a scope and a few feet of cable with it. Bottle opener towards mid Palm. Retractable bit driver. Different shaped pliers and vices. Knife, chisel, lighter, pry bar. Some kind of grounding cable and an insulator insert.

I want my robot hand to grant wishes.

Everything a tricorder can do: communications, biometric scans for disease/injury/etc, all the sensors, the ability to identify flora and fauna, 3d display/holographic projection of scanned environment and a built-in phaser would be awesome.

Ground penetrating radar is a high priority. Lidar as well. Mostly because I'm convinced my apartment is located next to or on top of an ancient native american temple that's been buried a thousand years.

The ability to 3d print stuff from any plastic waste i find would be cool.

calculator that can do the math i can't easily do (i.e. most of it - dyscalculia).

that thing cable from the x-men had that let him teleport like anywhere with a verbal command and was like wikipedia but from the future.

wikipedia, stored locally, with archived captures of original sources cited, including those from physical books which often aren't linked beyond a page number.

solar powered battery. i know i wouldn't really need devices that need recharging if the prosthetic had all the stuff i want, but i might need it to power ancient tech (basically anything more than like 5 years old thanks to planned obsolescence.)

a spellcheck that doesn't tell me when i spell words wrong, only tells me when i spells words right which i think i may have misspelled, like obselescence and misspelled.

everything cell phone voice assistants were supposed to be able to do, but never worked for me.

like voice to text (i have no teeth and VTT never understands me), setting reminders, actually reminding me of reminders i've set, accurate short term forecasting of local weather (especially a tornado warning system better than what my city currently has), capable of correctly answering questions about simple stuff like google search used to be able to do.

cloaking device.

body shield like from Dune.

This got long quick. I better stop here. I could keep going for awhile though.

Laser, torch and claw. The only real choice.

A knife!

For cooking and stuff.

Also like a whisk and a spatula and a permanent oven mitt like thing!

I'd look to Swiss army knives for inspiration. Knife, screwdriver, scissors, pliers. A light. Someone said phone holder.. I'd have the phone embedded in the arm, positioned to be easy to see and access. Have the lock screen be various health stats: heart rate, Vmax, Time. Usb charging port... Be able to both charge the arm, and charge devices from the arm, maybe with a retractable cable ala cyberpunk 2077. Camera in a fingertip, ideally connected to XR glasses, so I can see behind or under things easily, able to be a microscope. Electric arc lighter in another fingertip.

All I need is number three arm. RIP Mycroft.

Super high precision so I can manufacture computer hardware with my hand

The Zorg ZF-1!

That, or glow blue when a cop is near...

A 1/2 inch drive attached to a finger with a rattlegun in built. As long as i can keep the rest of grip and function.

An aprosthetic would be a non-prosthetic, like for instance an aphrodisiac.

Jerk my meat at top speeds, and have a built in fleshlight. Only things I would need.

Vibrate.

12 gauge shotgun concealed in it as well as several of the other more intelligent options from the other users in this thread.

Inject a neurotoxin discretely

An ion-engine beam launcher that I can charge with electricity. If not, then a 1 litre container. Feels good to wander without packages.

Definitely something something with AI. And a vibrating device.