Fabric: You need to check whether the fabric is 100% cotton, blended cotton, linen or some other fabric as the fabric should be moisture-wicking, that is, it should not absorb or retain moisture. The fabric should also be breathable and comfortable for the skin even for long durations. Another way to check that is by the GSM of the tee, which is essentially the grams per square meter of the fabric which shows the thickness and quality of the material that should be at least 160+ to last you all seasons.
Fit: The fitting of the tee should allow room for your skin to breathe and not cling to the body. We are ideally looking for something comfortable and relaxed, but you can also opt for a loose-fit tee that is super breathable but chic or even a slim-fit one which is body-hugging but comes in a breathable fabric.
Occasion: The time and place you are going to wear these tees the most is what will also decides the silhouette, because as good as an activewear tee is, you cannot wear it to formal occasions no matter how much you're capable of dressing up or down an outfit. If you will be wearing it most often during the day then make sure that the fabric is not see-through and also does not show any sweat stains. If it is more for office wear, then you can pick out higher GSM fabrics since most indoor environments are not as hot, and since it is high quality it will last you longer than any regular flimsy ones.