But wait: I already have found the world's best basic tank top: Lululemon's Power Y tank. It is expensive, and worth every penny. It solves all of the problems listed above, and more. I can wear them all day long, every day, and they stay put and stay comfortable. And no, I'm not paid by Lululemon to say all of this.
It looks much better on the Lululemon model, trust me. But my size 10 tanks stretched with me for nearly my entire pregnancy-- until the last month, when I just got too gigantic (I gained 40 pounds... and was 2 weeks late... but that's another blog post). To account for my larger bust once my milk came in, I bought 3 tanks in size 12 and added nursing clips. It was so easy to do that I did it two weeks after a very hard birth, while nursing my tiny baby in her K'tan wrap.
There are a bunch of tutorials out there on how to convert a normal bra to a nursing bra-- this is basically what I did. I found the tutorial by Christina and Janda at Mamas Worldwide very useful. I used clear plastic clips from The Nappy Shoppe, Style A. I used black 1/4 inch elastic rather than ribbon or cloth as my strap-keeper.
They are fantastic. Soooo comfortable, they stretch to accommodate very full breasts, and the "luon" fabric smooths over some of my postpartum bulges. Plenty of length. Great selection of colors and patterns. And an added plus-- the removable cups are awesome at preventing the inevitable leaks from showing through (and I don't wear nursing pads, even though I probably should).
My only pet peeve is that putting the strap back on isn't one-handed... the straps are a bit thick for the clip, so it is a snug fit. I don't find this to be much of a problem. Overall, I love my nursing tanks!