#1 - Lionel Messi:

Lionel Messi might have reached 30 years of age, but he is still in the eyes of many, the best player in the world.
The Argentinian has guided Barcelona to so many prestigious trophies over the past few years, and is considered as the Catalan club's biggest idol. After losing Neymar, who was supposed to be Messi's future replacement, it is practically impossible that Barça accepts to sell the Argentinian at any cost, even less so, if the transfer fee is inferior to what Paris Saint Germain paid for Neymar.
Messi's buyout clause is over 300 million Euros and his contract expires in June 2018, however, it is extremely unlikely that the La Masia product ever leaves the club of his heart on a free. He has stated on numerous occasions that he dreams to retire at Barcelona.
But with the current incompetent board in charge of the club, one would not be surprised if they ever considered the idea of offloading Messi in order to generate a huge amount of cash. In this case, the transfer of the 5-time Ballon d'Or winner would definitely be for a world record fee.