My perfect shirt would change every day. Not the fabric, of course, because once you find the perfect fabric you want everything you wear to be made out of it. That fabric would be a whisper light, amazingly soft, organic cashmere silk blend. It would hold up as well as waxed canvas and wash easily, air drying quickly without shrinking, always soft, never losing a thread, keeping an odor or attracting a hungry moth. But only all natural fabric, 100%. Yes, I’m willing to forgo dyes and keep it a natural color because the attention getting feature will be the one that doesn’t exist yet.
Now where my perfect shirt gets interesting and becomes something out of science fiction is the better than Organic Light Emitting Diode technology screen on the front (I’m sure your version of the perfect shirt could be customized with the screen on the back or sleeve). This screen would be made up of nigh magical biological bits and molecules that would allow me to change the design on my shirt at a whim. I would just link it to one of my local pieces of Apple tech and find a favorite tweet:

Or quote:

Or upload one of my own doodles:
Or for lazy days, my favorite comics would allow me to wear their latest punchline.
Yes, the perfect shirt would be one that suited the fickleness of my daily and/or hourly mood. Is there an app for that yet?