Those T-Shirts are expensive, but I personally find them worth the investment.
I find them to be the absolute perfect plain T-Shirt. They fit me perfectly, stay soft after being washed a lot (Had one for almost 2 years and it still looks perfect). They're thick enough to not be see-through, but not too thick. The collar doesn't get all weird after being washed a lot. They're soft as a baby's bottom. And the pocket just rounds them off.
They can be thrown under a shirt, or just worn by themselves. I love them.