On warm days you will find yourself wearing your top base layer alone. Other than your socks, it is probably the one garment that will suffer the most wear and tear.
Being a base layer it should be made from a material which is quick drying, will wick perspiration away from the body and is unlikely to chaff.
It is a good investment to go for a technical material (synthetic-fiber) that meets these criteria.
Avoid cotton, it absorbs and retains moisture.
We purchased Peter Storm base layers which have the added advantage of having been treated with a biocide. Consequently, despite being rinsed through only twice during the whole trip, they never developed an odor!