Atletico Madrid
The Rojiblancos are, without a doubt, the biggest challenge that Barca will face in the league this season. Fresh from a 4-2 humbling of their city rivals Real, Atletico could be looking at one of their most successful seasons this time around.
Antoine Girezmann's decision to stay certainly helps them while the singing of attacking midfielder Thomas Lemar from Monaco gives them a player who has the mentality and skill to do wonders under a coach like Simeone.
Club captain Gabi's departure may not have a significant impact as Atleti have roped in highly rate Spanish midfielder Rodri to replace him and the youngster put in a very decent shift against Real.
Other singings such as Gelson Martins, goalkeeper Antonio Adan, and striker Nikola Kalinic add more depth to the squad, something that manager Diego Simeone will look to make the most of as Atleti look to challenge for the Champions League as well, after a disappointing group stage exit last season.
Atletico Madrid, especially under Simeone, have been a side that has often relied on character more than skill or technique but with the players that they have this season, we might just see a mix of both.
And that is what makes them contenders not just for the league but for the Champions League as well.