For me it's liquid eyeliner. I'm blessed to be ambidextrous, so I can put it on with either hand, depending on which eye I'm dressing up.
Since I'm in my mid-40's now, I have begun to tone down my previous makeup style. Colors are less bold, eyeliner is less dark, and I'm more careful to blend. Nothing looks more clownish (except maybe a clown) than an overly made-up middle-aged woman. I keep thinking of Mimi from the Drew Carey show...

I've also switched from a cool palette to a warm one, and from frosts to mattes. Shimmery bubble-gum pink looks better on teens and twentys, IMO. I'll take the rusts and terra cottas.