https://www.health.harvard.edu/newsletter_article/supplement...
I take D and fish oil, and from my reading the science on both is strong. I also take 4-in-1 Fiber but people who have a better diet likely don't need it, but I find it beneficial (and, no, "number two" isn't the exclusive reason to make sure you're getting enough, fiber has numerous health benefits with "regularity" just being what is famous for).
Creatine also has surprisingly strong scientific evidence with few side effects and has been heavily studied in sports medicine. It is niche though, primarily used by people doing regular resistance exercises or pro athletes to help expedite recovery and repair.
PS - Creatine does cause water weight gain by design (it causes your muscles to hold more water) which can result in 8-16 lbs of additional body weight after two weeks~ and then for two weeks~ after you stop taking it (this isn't a problem nor body-fat gain, but people trying to lose body-fat need to expect this; it is largely a flaw in using scales to track body fat which ignores water weight/inflammation).
PPS - Creatine's COVID and post-COVID price increase has made it more niche than it perhaps once was. It used to be near $10 per 30-day from a reputable brand, now it can be $20-30 depending on brand (e.g. Creapure is $32/30 day right now).