Everyone is different. Neurotransmitters, neurohormones, brain structure, gut microbiome, and more make us all unique.
It also depends on how you want to use it. Some work great for a little while but are bad long term. Some are more likely to knock you out, so they're better for night time.
You might try valerian root, Ashwagandha, or a stress relief tea.
As refenced in your other post, ashwagandha works well for many people. L-theanine is another popular option. Its often combined with caffeine in energy drinks and pills. If you're taking caffeine already you might try adding L-theanine even if it doesn't seem like the anxiety is related to caffeine intake.
Cannabis works wonders.
Seriously - depending where you live or how easily obtainable it is - a small edible or a couple puffs is great for de-stresssing
That's a tough one, it'll be difficult to find a supplement that will have a dramatic effect on stress that's scientifically proven and not just placebo.
The best I can recommend off the top of my head would be Vitamin C, which has been shown to lower cortisol levels (stress hormone). However, this will only have so much effect on your overall stress, especially if the source of your stress is not resolved.
Being deficient in Magnesium, which is extremely common due to a lack of common dietary sources of magnesium, will also make you more susceptible to stress. A cheap 500mg Magnesium Oxide or Hydroxide supplement taken with meals will take care of that. And don't bother with more expensive complexed forms of magnesium, they have less elemental magnesium than oxide, and their quick absorption is not beneficial, as the human body can only effectively absorb magnesium at a slower rate.
I'd also recommend getting a blood test done to see what your Vitamin D levels are. Vitamin D deficiency can really fuck you up mentally, giving you dark thoughts, depression, and make your stress seem even worse.
Although ashwaganda helps with anxiety magnesium also helps better it this department of stress I believe
