What would you suggest to construct the ultimate deployment packing list?

Bring a little bit of cash. The unit you are replacing is likely to be selling off stuff they don't want to pack like TVs, game consoles, etc.

I know OPSEC and all, but on a scale of 1-10 where 1 is "cushy-live-in-a-hotel-in-UAE" type of deployment and 10 being "dig-a-hole-in-the-side-of-a-mountain-and-live-there-for-a-year" deployment, what kind of living conditions are you facing?

Because my answer will differ greatly.

laptop, and a 1+ TB harddrive with as many movies or tv shows you can get your hands on is a must to kill time when not on mission.

Really depends. Even if I knew the theater you were going, the specific, camp, FOB, or AB location within theater will change your packing list. My last combat deployment was much more austere and in higher elevation than all the other BN in the BDE. And btw, even then we didn't need any TP (would have been a waste of space). So here's my advice: 1) Receive guidance from your chain on the specific location. They will likely do a pre deployment site survey and can push out very helpful info. 2) If your location has an APO, you can order Amazon and receive in about 2-3 weeks. So travel light on stuff you can simply order. Also, honestly, you might have a PX close by or at least a PX rodeo that travels to your location. 3) Ask your chaplain what his plans are re: care packages, etc. I had a resiliency tent with more supplies from care packages than the PX rodeo. Troop didn't need to buy any hygiene products the whole deployment. Plus had lots of snacks. However, I took morale as a chaplain mission. Some don't. 4) Listen to the other suggestions about that 4TB harddrive for movies and TV shows. I'd also suggest a couple books. And, if MWR/USO puts out some of those audio books, get a couple. But bring a few pair of good and comfortable, corded earphones. This will likely help in flight and are more comfortable than the cheapos in the audio book.

Top-rate, A-one, multi-ply toilet paper. Great for filling in the space(s) in you Sea-bag (or whatever you use to pack your stuff. If you don’t use it, it ...well, used to be... can be leveraged as barter.

So my biggies for non mission essentials are Jetboil/BRCC pour over kit and a supply of coffee

Snacks on snacks on snacks( Also with snacks ramen because you have the jet boil)

Baby wipes

Extra skivvies

Laptop with Atleast 4 TB hard drive for movies and what not, can trade movies.

Books to read.

At least 23 pairs of socks, 17 t-shirts, and 30 pairs of draws. There will be times you just want to throw them away. Plus some will die concertina or similar deaths.
Cards.
Bible (or similar holy book).
Credit Card (for ordering all the stuff you forgot).
Leatherman/Gerber and plenty of 550.
Woobie.
Laptop with games on it, to include some "no brain" games like solitude for when you have time to kill, aren't physically tired, but are mentally fried.
Pen(s) and paper.
TP. Seriously.
A small stash of Pogie Bait. (This will need to be replenishwd, bit don't bring it all at once - it goes stale and/or disappears too quickly.

If you can hint what type of deployment, we can tailor our answers better.

I will say a lot of the time now, you will get enough internet long enough (even in a place like Afghanistan) to, if you need specific items, you can order it from amazon. This sounds dumb, but you get free shipping and you can bulk order stuff you need for your unit. Don’t overpack things you can get easily (like soap and stuff) for the love of all that is holy it’s a terrible experience.

I did not know this when I deployed, and significantly overpacked. As SSG (Join to see) states, the outgoing unit will be willing to sell anything to lower the weight going home.

Poker set is a great option. If you are geeky bring Magic: the Gathering or buy the decks online and have them shipped - awesome time wasters that don’t require electricity. Make sure you have a headlamp though.

A comfortable pillow! Baby wipes, towels, shower shoes, noodles lol, if you smoke or dip you’re gonna wanna bring your own brand, a tough box... man I could go on and on lol

You are all awesome thank you!