The v-neck tshirt. A classic piece that works for both semi-formal or casual occassions. I personally prefer to wear a looser white one with jeans and a pair of Ray-Ban aviators for the casual days. If I want to be dressed up and comfortable on a summer day, I prefer a black one that is a little more fitted.
More recently, I have been seeing a v-neck that is a little wider and deeper. It’s a good look as long as it isn’t too wide or deep. Whatever the case, a v-neck tee is a good call especially since you can grab some good ones for reasonable prices.