There is never a "best", there are only pros and cons.
There are many versions of PoD (print on demand) for shirts and such. Others are SpreadShirt, Zazzle, Mojo. But of all of them I would say that CafePress is one of the best FIRST places to do it.
A) The free shop is really free
B) the system for creating a shop and uploading designs is pretty easy to use
C) it gets the most net recognition in the most search services
D) the forums are a ton of help and other shopkeepers are willing to share their tips
E) it lets you play with the biggest variety of items and the pricing is good
F) all in all it covers the most areas and lets you really dive into it right away
On the other hand....
Other companys can have more styles of shirts in more colors, they might support bulk-orders in case a real-brick store wants a bunch of your designs, and some of the others have online customization so that visitors can make changes to your stuff on the fly then order it. However those places often lose you the "free", the ease, and the ability to order just one shirt. I consider them to be "move up" companies after you have proven that you can sell a thousand shirts in one order.