Here's a thing… It happens in cricket (leagues like IPL). The players get bid and sold at a much higher price even when they aren’t necessarily performing well in recent times. This happens because of the brand value that these names carry out of performances from the past. It happened with the likes of Yuvraj Singh, Dinesh Karthik and so many more. Even with Virat Kohli; he has been performing poorly consistently asRCB's captain, and yet the team owners continue to shell a ridiculous amount of money on him.
On the contrary, in football, except for a very few instances, this doesn’t happen. The footballers get the salary based purely on their performance and not solely on their name value. Players get transferred to different clubs, and are paid big, because they are performing incredible in their team.
For instance, Kylian Mbappé got transferred from Monaco to PSG in a deal worth whopping €180 million not because he has brand value. In fact, there's a good chance that you don’t even know him. He got that deal because he has been performing phenomenally for years for his team. This is just as true for transfers of players like Cristiano Ronaldo, Gonzalo Higuain, and Paul Pogba.
Point is, unlike in other sports, footballers get a salary based on their performance and not because of their name value. So, the ones who are getting paid more, they are likely performing exceptionally and contributing in their respective teams the most.
Coming to your exact question, it's too complex to answer accurately. Why?
There are millions of footballers in the world. At least thousands of them, if not more, play for international-level teams and are well-renowned. To that, most of their salary isn’t known.
What your question asks is to match these players' salary with their performance ("performance" itself is quite subjective; it's difficult to quantify the performance of, say, midfielders), and then come up with a list of players with the best salary and performance ratio. It’s difficult to come up with such list manually, in lack of sufficient data.
That said, here are the top 10 highest-paid footballers of 2019 based on monthly earnings:
3. Antoine Griezmann - £2.9 million
5. Luis Suarez - £2.5 million
6. Gareth Bale - £2.2 million
7. Philippe Coutinho - £2 million
8. Alexis Sanchez - £2 million
9. Kylian Mbappe - £1.5 million
10. Mesut Ozil - £1.4 million
(Courtesy: How Africa)
This list would give you a fair idea of the best salary and performance ratio of the footballers. If they weren't performing well, they wouldn’t be paid this high.
Hope this comes close to answer your question.