Carlos Sainz Jr.
For one and a half seasons, Sainz was partnered alongside the highly rated young Dutchman Max Verstappen. During their time together, the Spaniard outperformed his teammate on many occasions and especially in qualifying.
The fact that Verstappen has already moved up to a top team and is doing well alongside an established driver, therefore the chances of Sainz getting a seat with a top tier team himself have also increased.
During his short tenure in Formula 1, he has already proved his mettle through several strong performances even in an inferior car. But, it is widely known that Red Bull doesn't want to let go of the Spaniard, there are strong rumours going on about a contract extension for next year.
It will indeed be hard for Ferrari to sign Sainz and if they do decide to do it, then similar to Perez, they will have to let go of a lot of cash in this case too.