It's definitely not pants. Because that is the hardest item for me to fit. And you said "defines your style" .... I am always trying to NOT let my pants or jeans define my style, since I don't want anyone to realize I am wearing the same pants/jeans over and over again.
I agree, third piece is probably my defining piece. But it changes depending on trends. For a long while it was jacket or blazer or cardigan. Now... not so much. The trends have shifted. More minimal. It's more about the starring role of top (fluid relaxed half tucked asymmetrical boxy) or bottom (bright color; crotch drop hip hop; BF jeans; ankle; skinny sausage casings; skinny relaxed). This is a challenge but I'm trying my best...
In the meantime, DRESSES. So easy. How much more minimal can you get? It's one piece! No fuss, no muss!
With scarves and shoes and belts carrying a lot of weight playing the third-piece role.
I miss interesting jackets and cardigans. But they'll come back around.