Well, Zamorano and Salas really made an era. The former one being top scorer (pichichi) while playing for Real Madrid circa 1995 and then joining Inter de Milan, sharing the pitch with Ronaldo, Baggio, Djorkaeff, etc; the latter winning the Best Player in America in 1997 if I remember correctly, becoming an idol in River Plate of Argentina, and then jumping to Lazio and Juve. Both of them were the top scorers for the National Team until Sánchez came along and dethroned them. Now Alexis holds the record with 39 goals or so, and with a good few years in front of him to increase his record.
There's another historic Chilean player who always makes it to the Top 3: Elías Figueroa (CB). He played in the same era as Pelé and won 3 consecutive Best Player of America in the 70s. He's still regarded as the best local player in our history.
When Alexis moved to Barcelona in 2011, the argument about who was the best was reborn, and now Alexis and Vidal are easily in the top 3 of our most successful players, probably with Figueroa in the podium. I think that if he makes history with United, and he gets to win a couple of leagues and especially the Champions League (which no Chilean player has ever won on the pitch–Bravo won it when in Barcelona but as a sub), he will definitely go down as THE best Chilean player ever.