Got my first KSH last season. And am much closer much earlier this season.
I don’t really have a lot of older stuff. Didn’t get this account till MoP. And having a completely blank account after everything I did on my parent’s account took away a lot of motivation. So all I did was BG and level new toons. Didn’t really start doing content again until SL.
So I guess my proudest achievement is kind of ongoing. I feel like I’m coming into my own as a WoW player.
Sticking with this train wreck for 17 years 
When the reputation achievement was hard to do.
Falling whatever yards without dying by far. It’s the only achievement that really matters. Everything else is just meh, boring.
Probably the first time I hit 2400 in arena. I did it with the woman I was dating at the time, so it felt special.
Other than that, probably when I got Conqueror.
Same here!! Was fun!
That time I fought off and won against 3 Demon Hunters in Nazjatar.
I’ll never be gladiator, but in that moment, I was beast mode
Back in Wrath on my original server being one of the first Horde guilds to down Zero Light Yog, Heroic Lich King, and Algalon.
Also, I remember getting raids together to do a weekly raid on the faction leaders, even people who already had the mount would come. That was always a blast.
Getting all the Mage Tower (Legion) classes/specs done before they buffed the weapons (which I assumed the Mage Tower was going to be closed before they did it, always kicked myself in the butt because people told me it was easier when they did that.)
crazy cat lady i love that title 
Dunno. I’ve got a lot of stuff.
Being happy, not with the game, but just in general.
Way before BFA was released, I got an email inviting me to do Alpha and Beta for BFA. I leveled 2 Demon Hunters did all the leveling quests available. It was an experience I will never forget.
“Going Down?”
SKål!
Id say getting my charger for my pally back in the day, good lord was that quest a pita but it was worth it
A demon hunter that is best friends/exalted/etc with every faction/etc that is currently possible.
Black proto-drake a close second.
Two achievements, both done out of spite:
1.) In Shadowlands I was told that if I didn’t play fire mage I wasn’t playing the right mage. I picked Arcane Mage. The Achievement happened when Twisting Corridors was dropped. As it turns out, Runecloth Wrappings as well as stacking crit damage and chance meant huge, outlandishly oppressive DPS. By the end of the Twisting Corridors, I was two-shotting the end bosses. I ended up making hundreds of thousands of gold for ‘Maw Mount’ runs because word got around I was doing several million DPS. It was a good time.
However, outside of cheesing mechanics in a bad feature in a bad expansion, Dragonflight holds my best personal achievement. When Evokers dropped, I wanted to main one immediately. I decided to learn and practice Preservation Evoker, and not even a week into the expansion drop, I found myself in a random dungeon with a chipper, kind fellow from the Stormrage server. I had never healed before this expansion. I had never played Evoker before either. Everything was fresh and new, and because of this, I was not the best a week into the expansion.
This kind fellow told me to stop healing and swap to DPS.
I replied with a firm no, as I was still practicing, and if he didn’t like it, he could boot me.
We eventually cleared the dungeon, but I made a promise to myself that I’d never stop improving as a healer.
I got AotC and KSM last season. I got KSM this season, and soon AotC with my guild, probably even KSH next week.
Go to H-E-Double Hockey Stick, Mr. Stormrage Server, I am a great healer.
I’m not really a try hard. So when I do try hard stuff or even try a little bit stuff and succeed I feel good about it. My two try hards are my “of the Black Harvest” title and feat of strength along with destro mage tower weapons and fire mage tower weapons.
Mine would be “what a long strange trip its been” and “did somebody order a knuckle sandwich?”
Also “for the horde/alliance” love my black war bear even though i dont pvp anymore
Using goblin jumper cables to rez an undergeared healer in Halls of Reflection (when it was first released) in between waves of spirits and saving the run.
Soloing Tyrande in that timey whimey dungeon from 40% on my subtly rogue.