Granted I'm a "foodie" who rarely eats at fast food restaurants outside of WDW, but I have found the QS food I've had in WDW, and the whole, to be mediocre at best. Certainly better than most other theme parks, but nothing I would go to by choice if I wasn't a captive in the parks. There is quite a lot of variety available at QSs, although not necessarily all things in all places. Unless it's a "specialty" place, most of the QSs offer some sort of salad, wrap, sandwich, or some "healty choices"; they're really getting better at this. But they are "fast food" versions of these on a par with what you might pick up at a grocery or convenience store. Other than the WS locations (which I truly enjoy and look forward to), I would consider QS to be "sustenance" and not a "dining experience".
EP is the only park with "good" choices. Yorkshire Fish and Chips, Tangerine, and Sommerfest are all good; Katsura is not bad either. Sunshine Seasons probably has the best variety of any QS,and the food is better than average.
In the MK, Columbia Harbor House is about the only one I have liked, although the new menu (that I haven't had a chance to try yet) at Pecos Bill's has some promise. The cinnamon rolls at Gaston's are very good and are typically my MK breakfast.
In AK, Flame Tree gets very good reviews, although I have not tried it yet. Yak and Yeti is pretty good for "Chinese take-out". I almost always eat at Y&Y (TS) or Tusker House. Kusafari in Africa has some very good breakfast pastries.
Other than cookies and cupcakes, I don't know if there IS a "good" QS choice in DHS. Pizza Planet was tolerable in a Pizza Hut sort of way, but it's closed until Nov. When it reopens, I expect the food to be pretty much the same. The one meal I ate at the Commissary was probably the worst meal I've ever had at a Disney park.
I'm probably a harsher judge on QS than most because, as I said up front, I rarely eat fast food outside of WDW, and as 3 out of the 4 parks have very good to excellent TS options, I rarely rely on QS to get me through the day.