you gain the most experience as driver 1 and 2 as Driver 2 will soon lack experience if you always use your testdriver in The first Practice Session. Also The Drivers in your own Team do not seem to level faster than The CPU so you can always wait out until the Drivers are Strong enough for your Team

The way the game is setup, a reserve driver is just a dead spot. You will have far more success giving 2 drivers with reasonable growth potential car 1 and 2, if you want a longer career. If you want a short career you can pick drivers with already good stats - the growth potential does not matter in this case. The reserve spot can go to the cheapest driver you can find. He / she will be just gathering dust and look pretty while polishing your cars.

If you want to know which driver has the most growth potential at the start of the game (max is 100 - every season it is lowered by 2):
Jak Crawford - 98
Zak O'Sullivan - 96
Oliver Bearman - 94
Théo Pourchaire - 93
Isack Hadjar - 93
Zane Maloney - 91
Roman Stanek - 91
Franco Colapinto - 91
Oscar Piastri - 89

For reference: Fernando Alonso's growth potential in season 1 is 15.

I went for Piastri in car 1 and Pourchaire in car 2 for 11 seasons. At the end of season 11 both Piastri and Pourchaire had an overall rating of 95.