if you had to clear everything from the bank and bags and leave only 1 thing, what would it be ?
*no legendaries, too boring.
for me,
Dwarf Beard Rings
its a grey item that drops as 12. it is a reference to The Hobbit, which is one of my favorite books i read as a kid.
The Darkmoon Cannon, because for some reason it’s still not a toy.
Did someone say [Thunderfury, Blessed Blade of the Windseeker]?
Hunter epic bow from quest i got back in vanilla.
i got some of those aswell, im just waiting for them to make it a toy.
My original Haunted Memento… one day I hope they’ll un-nerf it.
This is a really good question!
Probably my Blessed Regalia of Undead Cleansing. Okay, it’s 4 items. But it reminds me of good times.
Just wish I had the Plate version.
I’ve got a couple of those still floating around. Gonna be keeping those. Although I do wish I could move them across servers to characters that I now play.
The [How to Make Money and Influence Subordinates] gray item. I still hope I can read it one day…
A male Gnome at Goldshire, wearing this shovel and nothing else = Great success 
Slimy Ring. My now irl husband give me it when we first started raiding together and chatting back in wrath. Good times!
Mostly just stuff I spend a long time trying to get and/or has sentimental value. Also the unobtainables from previous expansions.
But the most valuable thing I’ll never get rid of is a simple little [ Strand Crawler] pet my (now deceased) friend gave me.
The Pandaren starter robes you get while leveling in the Wandering Isle. I keep them in my bags since 2012, when I leveled Starney as a Pandaren named Sczechwan, just in case they become available for transmog. Now it’s almost time! I think I will keep them even after Dragonflight is released, just as a nostalgic reminder of the old days.
I also have a spare Raven Lord in my bank for no reason, but I don’t want to delete it. 
All my bags are full cuz im too lazy to delete things. Same goes for all my alts
Because I kept linking this item to my brother just to annoy him during Legion. Item describsh is golden.
Ashbringer
Bending the rules a bit as it’s currently 8 items but it’s one set:
All 8 items of the Feralheart (Druid Dungeon 2) set. It was quite a hard task at the time and my group in the end managed to get it done.
There is only one constant in the world, one thing that rules everything, one thing that fixes all problems… BACON!
Nat Pagle’s Extreme Angler FC-5000. Picked it up when levelling my first character (still my main) back in early 2018, and it’s been in my bags ever since.
I have the little Smelly’s Luckydo from back in MoP that I’ve seen sell for quite a bit of gold but I’ll never get rid of it on my monk 
Sadly it has never brought me any luck…there’s time yet I guess!
I would leave a bag in my inventory…