I have the all-time WORST movie ever made. I'm surprised I didn't think of it right off the bat:
"Battle Of The Bulge."
In addition to being completely inaccurate historically, it even had the premise that a Boston police detective saved the allies because of his detective work. On top of that, the German panzers were nothing but painted over Sherman tanks. Horrible acting by Telly Savales as a tank driver. Robert Shaw incorporated every silly German war stereotype into his character of Colonel Hessler.
If you have ever seen the "Battle Of The Bulge," then you have to agree that it was the WORST war movie ever made.
I didn't remember a black Vietnam flick by Singleton - when I look up his filmography, I don't see one in the past, nor among his future projects. I'm not sure which movie you're referring to...