The choice of a hiking t-shirt is based on 2 criteria: the intensity of your physical activity and the outside temperature.
The intensity of your physical activity
During a rather intense hike (that is to say with +/- elevation and sustained physical effort), you will perspire, so it is therefore better to choose a t-shirt made of synthetic fabric because it will be breathable and also dry quickly.
If you are carrying a backpack, we advise you to wear a t-shirt with a mesh back like on the opposite photo, as the fabric is more breathable.
For a shorter, less intense hike, you could opt for a shirt or clothing made of more natural fabrics. They are comfortable, but are not as lightweight and do not dry as quickly.
In warm weather (such as during intense activity), choose tank tops or short-sleeved t-shirts that are breathable and/or have ventilation zips.
In cold weather however, choose long-sleeved t-shirts either made of Merino wool (a natural anti-bacterial material that limits bad odors), or brushed on the inside. These trap more air, which helps keep the heat emitted by your body better and therefore keeps you warm.