Well, the good word has to be put in for 25's Hawaiian shirts, even if they aren't T-shirts in the proper sense.
That said, I liked SPAM's wordsearch shirt from last year. To the point that I traded for one.
I also recall 1002's bling shirts, and 1083's retro shirts. (Would retro in FIRST involve lots of corn jokes?)
We'll see which ones get the big trades this season.