What's your absolute Jank but favorite go to WAD?

I don't actually replay it often, maybe once each decade at the current rate, but Little White Mouse's Gunsmith Cats mod for ZDoom often comes to mind. It's another one that requires an older version of ZDoom or it just kind of breaks. It had this interesting Postal 2-like mini open world thing going on, which is why I think it sticks. I also have a soft spot for that kind of 90s~2000s autistic fanwork mod with various factors kind of locking it into a particular place in time. It has a frustrating mechanic where if you take a hit, you continually bleed out until you get some healing, which translates to lots of running back to the shop for heals and armor if you can afford it. I seem to recall being less enthused with continuing play the last time I fired it up. I wouldn't really recommend it beyond archaeology/research.

LWM was a very ambitious modder. Bittersweet was a pretty interesting effort which is probably memoryholed now for its very nsfw art, but when I realized what it was doing with the dynamic map, I was impressed. Less janky and more fun than the GSC mod, but it's more or less an incomplete proof of concept.

Lastly, it's not really jank, but Hybrid Fragments by lupinx-Kassman is an incomplete(still?) lightly-modded wad I used to replay a lot because it was one of the first I played after a long Doom drought, and most importantly it was the first wad I heard Memento Mori 2's Give In (With Pleasure) in.

I mostly try to avoid replaying stuff these days since my backlog stretches to infinity, but the GSC mod and hyfrag are always lurking in the back of my mind when I'm engaged with Doom content.

Herian, both I and II.

When I'm in the mood to breeze through a full megawad in under half an hour, I play this

Perdition's Gate definitely doesn't qualify as jank.  Those maps were amazing back then, and honestly still hold up quite well today.

EDIT:  With respect, I think the OP is a bit unclear on the term "jank" or "janky".  Janky games are broken, glitchy, or otherwise look as though they haven't had enough time, money, or effort put into them.  Take the Star Wars Outlaws for example.  That's jank.  Or a game like Chaser, Neuro, Kreed, or Cryostasis.