Bigger does not always equal better. The Backstreet Boys may currently be selling more records than Pearl Jam. But that does not mean that they are better. There may be more McDonalds locations than Wendys. But that does not mean McDonalds is the best. Far from it.
McDonalds is probably the leader in fast food because of its speed of service. But the speed also results in the food only being simply average.
Take for instance, McDonalds hamburgers. Ever notice how they taste rather dry. Not very fresh and not very juicy. That is because of the heating lamp approach McDonalds takes to food preparation.
What McDonalds does (and I have this on good authority from people that used to work there and some that still do) is make the burgers up in advance and then keeps them warm under the heating lamps until people order them. That definitely makes service go faster. But it also means that the burgers are not very good. Also, good luck on trying to get a burger without mayo (Or any particular ingredient you may not want on it). Because they are made up in advance, asking for a burger made a certain way will not go over well at McDonalds.
The rest of the food (With a few exceptions) isnt much better. The chicken nuggets are rather dry and not very tasty. Compare them to Wendys nuggets, which are very tasty and can be addictive. There is no comparison. McDonalds heating lamp approach strikes again. Also the fish sandwich is not as good as the Whaler at Burger King. The Whaler is a lot tastier. While the McDonalds fish sandwich is somewhat better than the burgers and chicken nuggets, it is still ehh at best.
Where McDonalds does okay in is in the quality of breakfast food. I only rarely eat breakfast there. But an order of hotcakes and milk usually hits the spot. They also have hash browns, Egg McMuffins and other breakfast related options. Also McDonalds French fries are definitely the best when it comes to fast food French fries. And the milkshakes are good.
While McDonalds is fast and efficient, the overall low quality of the food prevents me from recommending it. If you want fast food there are better options out there. Go with McDonalds only as a last resort.