The most effective supplement with the biggest impact I've ever tried is plain old magnesium oxide (the expensive forms are a waste of money, and you'll actually absorb less elemental magnesium from them. Just take the oxide form with a meal to avoid the shits).

It's difficult to get enough magnesium in most diets, and I found that ensuring adequate magnesium intake had the effect of making my mind just feel sharper (that effect is especially prominent if you're supplementing vitamin-D as well, since that tends to use up magnesium in its conversion process, IIRC).

nothing else has had such a noticeable, non-placebo benefit (except Vitamin D if you're deficient, which will make you crazy depressed without getting sun or supplementing).