From a storyline perspective, there's only one wunder weapon: the original WunderWaffe DG-2.
All the others are never referred to as WWs. Maaybe the Babygun, since Maxis designed it roughly around the same time as Richtofen did the DG2. And possibly the Raygun as well.
The term is highly subjective, so you can't say someone's definition of it is wrong.
The closest to a true and valid definition is the one @ came up with, it's very objective and thus it becomes easy to class weapons as WW or not. Props MMX.
I still feel the main reason people don't call the Jetgun a wunder weapon is simply because it sucks. (No pun intended)
If it was the best weapon ever everyone would be like "who cares if it's made from spare parts, look at the damage it deals. How can you not call it a wunderweapon"
Okay, almost everyone then. MMX would still hold true to his definition.