The vast majority of the grid are more or less of equal driving ability. The real difference happens in the drivers' heads and in the general team environment.
I think it's not necessarily about the ability but the style.
From Perez:
“I learned more about Max’s driving style and how the car has to be driven to extract the maximum from it,” said Perez.
“I think that was the biggest learning, I know what balance I need from the car throughout the weekend.
In other words the Red Bull is designed to suit the way that Max drives - not the skill with which he drives, but the way that he drives. The Mercedes favours Hamilton over Bottas because they will have slightly different driving styles - the way that Hamilton feels the car and makes the tyres last longer. Ricciardo's style is very different from Max, favouring late braking for example, which the RB may not be as easy to set up for.
The best drivers are those that can adapt their style to the car they are driving, or where the style is similar to the lead driver - hence Russell doing superbly well in the Mercedes suggests that he is either very adaptable or has a similar style to Lewis.