Simple one boys....who makes the best quality plain white tees these days? Well priced, good cotton, a neck that doesn't get all warped n droopy, etc.
Point me at 'em.
I did wear those Kirkland ones as undershirts for a super long time and they held up pretty well, never as an actual shirt though. Uniqlo is also good, price and quality considered. Probably could look there. And if you want to up it a little Everlane would be worthwhile to check out (and by upping it I mean go from <$10 to $15).