I found that some of the most comfortable maternity dresses available were maxi dresses. And that's very lucky since I was very pregnant for the summer time and maxis were appropriate and incredibly easy to wear. The one I'm wearing here is no longer available but I have narrowed down some of my favorite maternity maxis below. This Nicole Miller one is quite possibly my favorite - I wore it a ton to show off the bump!
Dress: old (similar here)
Shoes: Zara (similar here)