My favorite breastfeeding shirt is from Target [affiliate link], which should surprise approximately none of you.
It's a basic, soft, lovely short-sleeved tee that's essentially a flap in the front, and one in the back. I wear it over a nursing tank. The front flap can be lifted from one side to the other as needed and tossed over one's shoulder or used as a breastfeeding cover if desired while nursing.
It's marketed as a "sleeping shirt," but I caution against wearing it to bed – I got tangled up while tossing and turning.
$15 for the tee; $18 for long sleeves [affiliate link]. I ordered a size smaller in each since they're designed for bed, and got a perfect fit.
(One last word: if you use SnuggleMe swaddles, be aware that the velcro will snag this soft cotton and cause pulls the first time you wear it, which along with hormones may cause you to cry about how you can't have anything nice anymore.)