You can fill this thread up with Ecclestone's choices alone.
Replacing Wilson Fittipaldi with Richard Robarts/Rikky von Opel.
Replacing Carlos Reutemann with Larry Perkins.
Replacing Riccardo Patrese with the Fabi brothers. I maintain that Bernie kept on the wrong Fabi.
Replacing the Fabis with François Hesnault.
But the absolute nadir, and the biggest-ever mismatch between driver and replacement, was when Lauda quit the team, and Ecclestone installed in his place Ricardo Zunino.
Now, I appreciate Bernardo was basically stuck with any driver who had his helmet with him at Canada 1979, but Ecclestone then gave him the drive for 1980. This was a driver who aspired to mediocrity in F2. The equivalent would be Vettel walking out on Ferrari now, and Ferrari replacing him with Roy Nissany.
The extent of his out-of-depthness can be seen with his best result - 7th - coming at Canada when he was 4 laps down (OK, new bug, give him some slack), but then again at Argentina 1980. When he had had time and testing, at his home track, in a car that was capable of winning the title, yet he finished plumb last and was lapped by everyone. And he DNQ'd at Monaco. Game over.
Tyrrell also had some corkers, the no. 4 in 1980 had Daly scoring points and in 1981 Cogan not even qualifying, but they were almost always financially necessary.