looking for an anime thats straight up a master piece

Pico x CoCo x Chico

That is going to be entirely subjective and varies greatly from person to person, so it's hard to recommend something like that.

pelicula kimi no nawa una obra maestra de makoto shinkai

It's called "GOAT-ARM" for a reason. Anyone who says otherwise is wrong.
EXARM

I would say every show in my favorites list is a masterpiece, mostly cuz their strengths more that outshine their flaws. That being said
Ping Pong THE ANIMATION
Violet Evergarden
are the only shows ive ever given a 100/100 to.

Jujutsu Kaisen (Shounen), Overlord (Anit-hero), Usagi Drop (Slice of life)
Take in mind that I actually don't like Jujutsu Kaisen that much.

KON

LaidBack Camp

Mob Psycho 100
i see you liked one punch man, and imo mob psycho is even better

i personally liked the jokes and characters better in mob psycho, although i only watched one season of one punch man so... idk

What someone may say is a masterpiece is subjective so I'll just give u some recs that have potential to be a masterpiece for u
Monster
I noticed that u dropped it, but give it another try if u want to, it's one of the best animes out there, try to watch 3-5 eps a day if u watch more u might get burned out from the dense plot
I also rec
Neon Genesis Evangelion
Code Geass Lelouch of the Rebellion
Hunter x Hunter 2011
Fullmetal Alchemist Brotherhood
Perfect Blue
FateZero
Violet Evergarden
Mob Psycho 100

in my humble opinion
Mob Psycho 100

the second season especially

if you have a lot of time i would like to suggest one piece

Bloom Into You (My Favorite Romance & Read Manga after Anime)

A Place Further Than the Universe (Most Inspirational Anime Ever)

SteinsGate
Fatestay night Heavens Feel I presage flower (HF slaps)
Attack on Titan
Bakemonogatari
Clannad After Story
Mob Psycho 100 (Second one is even better)
Violet Evergarden

the term "masterpiece" is subjective but these are all extremely good imo

Yu Yu Hakusho Ghostfiles

anilist.co/anime/101291

Neon Genesis Evangelion

To the right people it is. I think it's overhyped and find it annoying that I have to do outside research in order to understand the story fully, but I think it's still good enough without that to be very high.

Since you love Attack on Titan as much as I do please watch this show. It has two of the best intros you will hear
``VINLAND SAGA

Legend of the Galactic Heroes
I'm surprised no one mentioned this.
Cross Game

Kaguyasama wa Kokurasetai Tensaitachi no Renai Zunousen
Yuru Camp
Nichijou
Gochuumon wa Usagi Desu ka
JoJo no Kimyou na Bouken

Abunai Sisters
no like this is actually straight up gas

Master Piece The Animation
Literally the name, my dude.

Keijo
unironically

KON Sound Euphonium 2 Liz and the Blue Bird

I forever stand by K-ON, Hibike & Liz

I can neither confirm nor deny...

It's hard to beat Golden Kamuy

i wouldnt call them perfect but these r great
Kimi no Suizou wo Tabetai
Hibike Euphonium 2
Violet Evergarden
Liz to Aoi Tori
Dororo

Here, no questions asked:
Ashita no Joe 2

(But you'll have to watch the first season up until episode 54 first)
Ashita no Joe

(Or read the manga until chapter 78)
Ashita no Joe

Mushishi
Hajime no Ippo THE FIGHTING

Clannad After Story

The Devil is a PartTimer
Don't ask me why just agree

just gotta be that guy lol

Assassination Classroom