I've noticed that either dresses or separates work well, but they have to be button down types. It's easy access with buttons, I feel. It's a popular option with my colleagues too. At my old work place, we wore uniforms and those were button down tops as well and my then mommy colleagues never had any trouble pumping at work.
For me dresses, buttons- type tops and tubes/spaghetti tops works for me. I have a nursing room in my office so it easier as I dont need to worry about being expose unnecessary. =)
For me dresses as well Easier to access haha. Also you can wear blouses with buttons or those stretchable tops would work fine.