What one (1) item would you suggest that a Service Member add to their Civilian Wardrobe?

Business suit.

Through my experience: Ties, maybe I just like ties, but this will allow more versatility when rotating your shirts and slacks:
- Shoes: Black are always welcome, think about adding brown, burgundy, and well cushioned dress shoes.
- Belts (brown/black)
- Brass collar stays (these can be purchased at Men's Wearhouse, they also have free plastic ones at request).
- Metal Collar Extenders (This are great in case your favorite shirt neck size shrunk a bit or your neck grew a bit).
- Handkerchief for suit pockets (Variables, kits are sold at Men's Wearhouse)
- 2 Black suits
- 1 Navy Blue Suit
- 1 Grey Pin stripe suit, grey goes well in rotation
- 1 good wrist watch
Always make sure you have a look that is well put together and you look on point. You are your own brand, and 1st impressions are important.

Nice leather semi-dress shoes. Sneakers don't cut it when you want to look your casual best

Loafers. Good ones.

They're multipurpose, can match anything, and if you have a couple sets then you can differentiate function based on how worn they are and what color they are. (the faded and worn ones for casual eveyday use, the still preserved ones for "business casual" or semi formal wear)

And they are comfortable!

I like cruises but will refrain because of COVID. On formal night I wore dress blues to represent veterans and military folks.

At least one pair each of black, brown, and cordovan leather (upper and sole) shoes with leather belts to match. Your belt should match your shoes, and your feet will thank you for quality leather soles on your shoes.
Wool winter overcoat and quality rain overcoat, if suits will be regular wear. Large, telescoping umbrella with wind vent.
A copy of "Gentleman: A Timeless Guide to Fashion."

Of course suits, nice shoes, etc... but the biggest thing that affects all of those - the knowledge of how to wear it.
Know how to tie a tie in something BESIDES a 4 in hand
Know how to tie a bowtie - don't wear a clipon (bow or neck) unless you're a cop
Know what French cuffs are and what you need for them
Know which buttons on your suit get buttoned, and which don't (hint don't button the bottom one)
Know how to add flair or "personalize" a plain blue or grey suit - ie socks, pocket square, tie clip, etc...
Know that the front pockets of your suit coat ARE SEWN SHUT and are probably functional if you remove 1 thread
Some vests are better with a neck tie, some are better with a bowtie, know when to wear each
Too many sets of stripes is a thing. ie shirt, tie, and suit shouldn't all be striped
Know when to wear a bright red "power" tie, and more importantly, when not to
There are fashion suits, and business suits (and most importantly, leisure suits!! lol) - don't mix em up
There are coats, jackets, parkas, windbreakers, etc..they are different things. Don't wear a jacket with a suit, suits get an overcoat
Once you leave the military you don't just change a couple things and now your suit equals a tuxedo.

Females the same applies, know your stuff, but you usually get more freedom with clothing.

Knowledge is what you need for wear, just like following the regs for putting you uniform together. An uneducated person in his first suit stands out big time.

How about some nice dockers shirt and shoes. foe relax time jeans and a t shirt

Burn all of the affliction tshirts if you were in the 82nd