Honestly, I can't stand shopping for clothing, you can waste an entire day and not find anything that's "both very comfortable to wear and [...] also flattering" and making people take you seriously.
With regards to comfort: For me, cotton, nylon, and silk are good materials. I am okay with rayon. I avoid polyester like the plague, which is hard because so many things are a cotton-polyester blend. I wear a lot of knits, something men generally can't get away with for a business look but something I find is wonderful in terms of comfort and a woman can it wear to work. I'm just careful to get the right weight of knit material that works for me/my look. That will differ from person to person.
I don't get your remark about making people take you seriously. Do you mean the sales people?
On the other hand, my 22 year old son gets misread as being a lot younger than he is and he doesn't hesitate to take advantage of such assumptions. Just tossing that out there as food for thought: Maybe it isn't only a problem. Maybe there is also an opportunity in there.