Versus their teammates
The one true measure of your performance in racing is how you compare to your teammate, afterall, they have the same equipment and personnel as you. It's fair to say Marcus Ericsson and Brendon Hartley aren't generally considered as bein two of the best drivers in the sport, but they're still some of the best in the world, you have to be to make it into F1.
In terms of qualifying, Charles is ahead a staggering 13-3 this season and during the races, Leclerc is 8-3 ahead in Grands Prix where he and Ericsson both finished. Gasly, meanwhile, is also 13-3 ahead of Hartley in qualifiying, and 4-2 ahead in races where they've both finished (STR's reliabilty has been poor this season). Even though Gasly has only scored points in 2 mores race than Hartley, Pierre always scores big, and that reflects in the drivers' standings.
Both drivers are beating their teammates all ends up this season, but I'd just about give Leclerc the edge in this respect, as Ericsson has improved this season and Hartley is still relatively inexperienced in Formula 1.