Other than my standard white Hanes crewnecks, which I wear almost every single day, I heavily rely on solid (usually black) T-shirts. In the winter, it's long-sleeve and usually from the Gap. They last forever, and when those things go on sale, I just can't help myself. (Still, I can't bring myself to purge many of those.)
This one's from J.Crew, which also churns out some great Ts, as you may have guessed. Their garment-dyed ones (green, please) are ace, as my pal Will from Bright.Bazaar might say.
The summer will usually find me in black, green, or something striped. I tend to stick with the G. BI part of ROY G. BIV. No Tuscany for me, please.