I pretty much keep coming back to these for plain black T-Shirts.  I usually end up spending between $15 and $25 on a 4 pack (depending on if I can find a sale when I need/want to purchase them.)  The material is very soft, and to color holds for a good while.  I expect the color to stay for a good 8 months to a year with repeated washing.  These cost next to nothing, and yet they seem to look good in most casual situations (throw a sport coat on, or a button-down with the top few buttons left undone to dress'em up a bit, if needed.)

FYI:  The large fit me best.  I'm about a 43short when it comes to a suit jacket, and I'm relatively fit. (Only including this to give a guideline for other buyers.)