The t-shirt. It's soft, it's comfortable, conveys your personality, and if you play your cards right, can be worn at least twice before it needs to go through the washing machine.
I'd even go as far as to say that in a hundred years, in 2112 (beware the Priests of the Temples of Syrinx!), folks everywhere will still be wearing the basic cotton, screenprinted t-shirt. I welcome your guesses as to the imagery in the comments below. Etsy Dudes, the guy-centric email newsletter of handmade supersite Etsy.com, dedicated their latest issue to that man standard (mandard? no...): the t-shirt.
They've got the standard: bikes, mustaches, sloths playing ukuleles, as well as maps and topography, laser pistols, and plenty of mid-century funiture silhouettes.
Overall, well over fifty Etsy shops producing great hand printed casual gear for guys. Totally worth a browse.