They're not cotton, but Majestic Filatures makes the best t-shirts around, in my opinion. They sell them online at Neiman Marcus.
I'm not a fan of MF myself. Mine get holes in them, which is rather annoying on a $200 t-shirt!
Just realised there is a whole thread dedicated to this! I must be living under a rock!They are French, you can buy them on their website once you know your size. Mine did NOT cost $200 per tshirt and last for years of constant wear. http://www.majesticfilatures.com/en/