A Monochromatic white pocket square or pochette, with edges, to go with any outfit, formal or not.
It gives a boost to your style and a touch of distinction while it tells to the other people “I am stylish and sophisticated”.
Silk is the most elegant but you can find it from cotton to wool to linen too.
Combine it with the color of the shirt or tie, the same color as your tie but of a slightly different shade, unless the chosen color is white.
Express yourself with a handkerchief, choose material, color and form (straight, triangle, puffy, etc.)
P.S. Use a different handkerchief for nose blowing…