I use an office chair that has arms that flip up out of the way when I don't want them there. I tried a few different things over the years - stools, saddle-style stools, balance ball...and I always come back to a standard office chair. I really like the one I have now, I think I bought it from Staples.
My machine sits flush in the table and the table is adjustable, so I have it set to a height where I can sit comfortably with my feet flat on the ground and my machine bed at elbow height. This makes it all pretty comfortable for me.
I have laminate flooring in my sewing room, so I bought a low-nap rubber-backed rug to use as my chair mat - like what you might see in the entrance to a store. It protects the floor, keeps me from rolling TOO freely (but I can still roll if I want to) and the little bit of carpeting helps catch dropped pins rather than having them bounce into invisibility like they did when I had bare flooring under my table.