Eyelash. Hands down.
I first used eyelash yarn when I was still just a crocheted, not yet a knitter. I had no idea what hell I was kicking down the doors of. I couldn’t see my stitches, everything was a knotted mess, and it was impossible to rip out.
I tried a different, similar fluffy yarn when I started knitting. While I was more successful, it still wasn’t great and the payoff wasn’t worth it.
I have worked with some other yarns that weren’t great, but it’s mostly that I have grown to dislike 100% synthetics and loosely plied yarns. Nothing comparable to my eyelash yarn nightmares.