I like the soft stretchy kinds that you just pull up or down (no snaps at the straps; no underwire), and my favorite is
Gilligan and O'Malley from Target. I am currently wearing the Large size, and it's a tad roomy. I was in Smalls before I got pregnant, and Mediums before that, back when Guinevere was nursing on demand still. So I have all the sizes.
I will stay in Large after baby's here and milk comes in (nice stretchy bras accommodate huge bowling ball breasts better, IME, for those very early weeks, plus you can sleep in them).
Once my breasts start going back down a bit and I'm losing weight, I'll go back to Mediums, then once everything's more efficient (around 9 months or so, likely), I'll be back in the Smalls.
Probably.
I normally wear a 34B but am wearing 36C bras with extenders (so a 38? no clue). If I were buying now for nursing bras after birth, I'd get a 36C, but I wouldn't expect to wear those bras until after my milk had come in and things had calmed down a bit. I'd wear the bigger stretchy/sleep bras initially (I get clogged ducts if I wear anything too snug underneath or with underwire).
But the stretchy bras tend to run small, overall. At least the ones from Target, Kohls, and Motherhood Maternity do. Motherhood has some in much larger sizes though, at least.