2. Lionel Messi, Luis Suarez and Neymar
It would be egregious to give an introduction to these three. Luis Suarez completed the set after his transfer from Liverpool in 2014.
Because of the FIFA ban received in the World Cup, he only began playing in October. In the beginning, with Suarez stationed on the right, the attacking play was ordinary with there a feeling that something was missing from the side, considering Barcelona had protagonists from the top 3 South American countries. After a loss to David Moyes' Real Sociedad, it was as if a switch had flipped. Suarez moved to the middle with Messi on the right, the Catalans exploded as they steamrolled their opponents, winning the treble in 2014-15.
Their outrageous talent and great relationship off the pitch translated to some sublime play on a regular basis. It was difficult for defences to figure out a method to stop all three at once. While they did not win nearly as much as they could have, they scored 364 goals and provided 171 assists in 3 seasons together. Individually, Messi led the way with 153 goals with Suarez just behind on 121, while the Brazilian managed 90. The sheer numbers are unimaginable. When you factor in the ingenuity of the attacking play, it amounts to so much more than mere statistics.
Neymar is a player most similar to Messi in terms of impact on the game with respect to goals, assists, dribbling and involvement in the team's build-up play. With the two on either wing and Suarez working tirelessly through the middle, it was a match made in heaven. That all three were lethal finishers can be seen in the end product.
The group was broken after Neymar's move to PSG in 2017. It almost happened again as Barcelona tried re-signing Neymar this summer. Assuming the transfer materialises and Suarez lasts one more season as a starter, we will be fortunate to get an encore.