there are a few episodes that have been criticized by some viewers and scientific enthusiasts for their departure from scientific plausibility or coherence. Here are a couple of examples:
"Genesis" (Season 7, Episode 19): In this episode, the crew members of the Enterprise begin to rapidly devolve into primitive forms of life due to a synthetic T-cell formula. The concept of devolution into previous evolutionary stages within a short period of time is not supported by scientific understanding.
"Force of Nature" (Season 7, Episode 9): In this episode, the warp drive technology used by starships is said to be damaging subspace, leading to potential catastrophic consequences. While the concept of subspace is a recurring theme in the Star Trek universe, the idea that warp drives can directly harm subspace has been criticized for lacking scientific grounding.
It's important to remember that Star Trek is primarily a work of science fiction and often takes creative liberties with scientific concepts to tell compelling stories. While the show often strives for scientific accuracy, some episodes prioritize drama, character development, or exploration of philosophical themes over strict adherence to scientific plausibility.
Ultimately, the "most scientifically nonsensical" episode may vary depending on individual perspectives and interpretations of scientific principles. It's worth noting that entertainment value and storytelling often take precedence over strict scientific accuracy in fictional works like Star Trek.