My Black War Bear. Mailed to my Rogue from Thrall back before the Cataclysm changed Org.
We were all stood around in Org doing City Defender and we collectively, not in a raid, decided … Yeah! If they are busy here, lets go do For The Horde! and we did, successfully in one swoop.
One of a few memories that have stuck, even after memory loss issues.
-Most Valued items:
Tabard of the Protector, and ZG tiger behind that.
Tabard is much, much more valuable to me than the tiger, though lol.
It’s simply one of those items that’s mega-rare at this point, everyone that had one quit years ago.
I have seen one other player in TWW that has it, and we immediately ran over to each other and started talking and fist-bumping lol.
-Most fond memory?
Me tanking Icecrown Citadel 25 on my old troll warrior named ‘Dinosaur’ in WoTLK, on a gaming laptop, at my friend’s house, while he’s playing a boomkin.
We’re on Deathbringer Saurfang, right after Gunship battle.
I tell him to take it easy on the threat, I see him rising up on the meters- the fight has barely started and he’s going pedal-to-the-metal on DPS.
Tank swap happens, threat is ripped off me right after I taunt, boss makes an immediate line for my boomkin friend, 1-shots him, and runs back over to me. All in like 1-2 seconds, lol.
We just bust out laughing and he watches the raid down the boss from the floor.
I told him to take it easy! 
Playing with IRL friends simply was the best.
For me it’s when I was still trying to play my Rogue, and Blinkstrike dropped during a Kara run.
My mind exploded and I said okay punks let’s roll!
But I was stopped, no roll needed and I was given the sword as everyone knew that’s all I really wanted from Kara anyway.
Was super cool of them all, but Blizz must have seen this wholesome moment as Blinkstrike was nerfed within a week or so and I got told to play my Priest anyway, as I truly sucked at playing a Rogue, fancy sword or not. 
Lastly getting “of the Bloodthirsty” title for 250k HK’s in MoP was pretty cool.
Karazhan was amazing, I’ve got a lot of memories in there too.
If they revamped it for a 3rd time, I’d still be excited for it.
Fun question. It’s weird to answer because the games been around so long, that nothing I own is particularly “rare” anymore.
But my first thoughts are things back in TBC and wrath that we’re fairly uncommon, but long(ish) grinds. Probably doing the Netherwing dailies is peak earned rewards for me. Searching for eggs, Booteranging, etc.
I’m hoping to get into a raid and get mine before reset!
I had to think about this. I think that for me, the most personally valuable thing I have in WoW is the first character I ever created still sitting here. My first main was a mage, and she’s still sitting on the server where we played in her 6/8 Netherwind and a staff she got out of AQ.
BUT the first character I ever played on was a vanilla human rogue. She has been moved servers and race-changed and name-changed over the years as my guild went from alliance to horde and back again…but she’s now sitting with my other frequently-played toons. She is a human again, and I chose all the same customization options I chose back in 2005 for her so she looks roughly the same.
In her bank, there is a 20-stack of Thistle Tea I made after getting the recipe in Vanilla, and she still has her Certificate of Thievery and her Thieve’s Tools.
To me…that’s pretty valuable.
green fire. i wasnt old enough to play wow when green fire also gave the title, but i love shooting my mtn dew flames at people and is a big part of why i main warlock now
The love rocket cuz it took me almost 15 years to get, and the mt dew mount that my friend got me
The Prot Warrior Mage Tower tmog back in Legion.
I’m not the best, nor am I’m terrible. But boy howdy was that encounter hard. It was RNG dependent and at times, it would just outright throw everything at you at once. Ghost going after the NPC? Meanwhile he does the leap on top of you, and everything else that happens at the same time. The bad thing about it is that Warrior had no reliable way to heal themselves, unless you had the Legendary bracers, which I did not have for the challenge. I must have done it well over 30 times before I got lucky enough to clear it. Another thing to note is that each challenge was tuned differently for each tank. And warrior was considered to be the hardest. Just getting that Tmog felt like an accomplishment.
Fyraleth was my first legendary that I obtained while it was still relevant to the xpac. It will never leave my warriors bag and will always be transmogged as my weapon.
Amani Warbear. It’s a reward from the first timed dungeon/raid, which seems to be a concept that is doing a lot of work these days.
My dreadsteed summoning supplies.
Honestly, I don’t know. Probably the year 15 balloons or my combatant title I acquired in BFA coming from someone is horrible at PvP
For me it’s this junk (grey) item.
In Vanilla, one of my best friends would send this to me as a rib, every time I caused a wipe. And, to this day, I’ve held onto a few copies of this item as a memento to all my early failures.
Two things…my first epic drop, a Krol Blade when I was grinding in the Winterlands(Sorry, Winterspring). Also my Corrupted Hyppogryph Mount I got from the CCG…
I had to kill H Fyrakk 15 times to get my Fyr’alath.