We got a great "Ask Jilly Moo" question about what our favorite tanks are. We both really love James Perse tanks. We both wore ours throughout our pregnancies and they survived the stretching out and they popped right back into place (can't quite say the same about our bellies). They come in tons of colors, are long, soft, thin but not too thin and better yet, have lasted through years and years of washings. There are several styles but we especially love the long ones. They are a tad on the pricey side for a tank (around $45-$55) but defiantly worth the investment in our opinion.
We haven't found a cheep tank that we like better so we can't really give a good recommendation as far as that goes. We have tried Gap and JCrew without much success. Just make sure you get a tank that has some stretch and isn't 100% cotton. We actually like the men's Hanes 3 pack of white ribbed tanks pretty good for layering and knocking around the house in as well.