The classic T. It seems like such a basic piece of clothing, but the cut of the shirt can make or break your look, particularly if you are a woman of a certain age.
This one by James Perse is a good choice, because the V-neck visually elongates the torso and the winged sleeve flatters the upper arm. (I once read that Sharon Stone pulls on the sleeves of her T-shirts to stretch them out and give them that winged look, because it makes her upper arms look smaller.)
This style by Nation has an A-line shape, so it’s a good choice to wear over jeans. I’m sure you’re familiar with the lumpy-belly look you get when you wear a fitted T over jeans. Since this shirt hangs a bit loosely over the bulk of the fly, problem solved.
If you don’t have biceps worth flaunting, an elbow-length sleeve, like the one on this Michael Stars T, gives you just the right amount of coverage.
Unless you pump iron on a regular basis, have great genes that make you look like you do, or have very slender upper arms, avoid a cap sleeve that ends at the fullest part of the arm, like this:
If you want to add a feminine touch, the ruched back on this Ella Moss T shows off your waist and takes the look from gender-neutral to girlie.