18. Bacon, egg & cheese bagel
As a New York person, I take my bagels seriously. I've actually maimed a man for asking me if I want it toasted (I don't), and I avoided jail time because everyone agreed it was just cause. The BEC portion of this sandwich is fine, but the bagel falls a little flat. It's not horrible, I'm just used to so much more.
17. Sausage McMuffin
Let's admit it -- though the McMuffin is a classic, it clearly has the weakest of all the bun options outside the bagel. The take on the English muffin isn't bad, per se, but it's not as good as the other varieties available. You can get the same stuff on a better bun. A bare sausage patty with a slice of cheese isn't enough to buoy this sandwich.
16. Egg McMuffin
See above, but sub in Canadian bacon.
15. Sausage McMuffin with egg
Again, just re-read #17, without the last sentence.