I would have to say that Sephora MicroSmooth face powder in 05 porcelain is the most valuable item in my makeup bag.
It is a full coverage powder that stays put, and keeps everything else (foundation, tinted moisturizer, concealer, etc) in place all day long, even under harsh stage lighting, heat, humidity, etc. And it looks very natural. I love it!
It’s $22 at Sephora, and worth every penny. It is by far the most needed and flexible item in my makeup arsenal. I swear by this stuff- it always lasts through my rigorous testing, and will definitely work wonders for anyone else.