Gildan's 5000 heavy cotton T-shirt is one of the most popularly used base garments for printing, across the board. 
Here's what the internet says about this product.
 
-5.3 oz./yd² (US), 8.8 oz./L yd (CA), 100% U.S. cotton
-Innovation you can feel. Made with 100% U.S. cotton and the latest breakthrough in soft cotton technology, the Gildan® Heavy Cotton family has been remastered for improved printability, quality and comfort you can see and feel.
-Classic fit
-Classic width rib collar
-Taped neck and shoulders
 
Responsible Materials: contains US grown BCI cotton
Responsible Manufacturing: Meets OEKO-TEX Standard 100 requirements and the company that makes this product is Fair Labor Accredited.