I've tried a few different ones and my all time favorite is the Glamourmom tank (
www.glamourmom.com). I started off with ones I got on ebay for $5-10 and have gotten the "color of the month" for $24.99 (which is the most I've paid for one). I don't have very large breasts, but I do need support and I find that these wear well too (the ones I got on ebay are ones I've had for 5 years and they're in great shape still!).
I have a second favorite -- which I have two of -- and I'm trying to find the name of it ... I can't find the name of it and since I hardly ever wear them (it's probably been over a year since I took it out!) I don't know where it is. But, I highly recommend the glamourmom tanks! You also get points and extra savings if you sign up for their glamourmom club. I wear a tank every day (and I've been breastfeeding for over 7 years so I've been through lots of different nursing bras and tanks).
ETA (again): Here's the other nursing tank, the Corsierre:
http://www.corsierre.com/html/product/index.html. It's VERY supportive. A friend got several on ebay and I bought a couple from her (she was trying different sizes to see which worked for her) and given the price point, I would try to buy it second hand or on sale. My friend is more buxom than I (more of an E/F cup) so it was a bit "too much" for me, but I like it otherwise.
ETA: Glamourmom has two different styles with one being more supportive than the other: the nursing bra TOPS and the nursing bra TANKS. There's definitely a difference in how supportive the two are.