The discussion in the "worst team-mate" threads created an idea. Maldonado-Senna from 2012 gets mentioned there a lot for not delivering on the car potential in good cars. So which seriously good cars have been wasted?
My point is more like about cases, where both drivers (and/or team also contributed to lack of score) underperform heavily, so you can't even be sure, how good that car was! So I do not mean the likes of Frentzen-Hill in 1999, where Hill was obviously nowhere, but Frentzen still delivered a lot. Or like Alonso delivered the max from 2008-09 Renaults, while his team-mates were nowhere. Because in those instances the car potential was still delivered by one driver at least and we could roughly see, how far up in the standings could that car finish in the championship.
Another one sometimes mentioned is Perez-Kobayashi from 2012 Sauber. In this case I believe team errors (and unluck) also contributed to the lack of score, as IIRC there were quite a few strategy blunders. In addition to the strategy genius in Canada and Italy.
And then... what about 1986 Ligier? Ancient 42-year-old Jacques Laffite and Rene Arnoux. Laffite drove only for half a season by the way, and still outscored Arnoux. What was the car's potential?