What are some of the hardest wads?

Yes, I've searched for old topics already, but they're not what I want, plus they're pretty old already.
I haven't been keeping up with the community in a while and blindly trying out dozens of wads seems a bit daunting for me.

I was wondering what levels are considered hard, but without them turning into a slaughter fest. Preferably without them abusing monster closets and traps that only work once. I want it to be as hard no matter how many times you've played it, not allowing strategies that make the map trivial.
Rather than just curiosity, this for studying purposes. I want to study them and find out what makes a good encounter that is truly challenging.

Chord Series

Valiant, especially the latter half of the megawad

Sunlust (Difficult all across, only a few slaughter maps)

Resurgence (although some maps are considered slaughterish)

Rush

Urania and it's sequel, Adonis

Those are the first hard-but-not-slaughter wads that came to mind off the top of my head.

Skepland

24 minutes ago, Kore said:

what makes a good encounter

Mostly personal taste, and decent execution on the mapper's side

24 minutes ago, Kore said:

truly challenging

mostly your own ineptitude

try "dimensions" map 02 and map 03. You'll quickly see why the maps are hard:

tight health, effective enemy placement, abundant use of higher tier monsters, limited firepower, sustained threat-levels.

https://www.doomworld.com/idgames/levels/doom/Ports/j-l/kewk

Dimensions by Killer5? I'd normally give it a try but it's not the kind of gameplay I'm aiming for.

Just now, Kore said:

Dimensions by Killer5? I'd normally give it a try but it's not the kind of gameplay I'm aiming for.

You said you wanted hard, but ideally not slaughter, map 02 and map 03 of dimensions are exactly that, sans one fight in map 03 that is mandatory, and one secret fight that is optional.

Every wad on nightmare.

Stardate 20x6 and Stardate 20x7 definitely rank up there in my opinion, and pretty much everything else by Ribbiks haha.

Check the Saturnine Chapel

Check out @xdarkmasterx's maps. I think they fit your description. Here's one of the newest, for example:

There are a bunch more single maps posted in their own threads; I suppose this is the best way to find them: https://www.doomworld.com/profile/20897-xdarkmasterx/?do=content&type=forums_topic&change_section=1

The Eye by mouldy. I have played it through on UV with all kills, without freelook (because i never ever use it in Doom). It's one of hardest my played maps. but i think it's not slaughter one.

Perhaps a map with branching paths/scenarios and optional slaughter fights.

Miasma by tourniquet, for instance.

You are almost all covered with the previous posts I think. I guess among wads as old as the Chord series, there is Crusades and Slayers.

Edit: and gggmork! How could I forget his maps? Try something like crapmap or impseV2. They are barebones visually but the gameplay is genius.

Check Death-Destiny maps.

Thanks for all the recommendations, guys. Some of these exactly look like what I was searching for.

Indeed Death-Destiny might qualify.

Swift Death perhaps? It's described as "Super Meat Boy meets Doom", which is kinda accurate.

2 hours ago, Kira said:

Crusades and Slayers.

Seconded for the "high difficulty with low monster count" category, also of note are his final maps in the Monolith series

Magnolia didn't have too many slaughter encounters from memory. Definitely worth checking out. Swim with the Whales also doesn't have a huge amount of what I'd consider slaughter, but some people would consider the fights to be too high in monster count I guess.