Their strongest challenger still belongs to Barcelona at this time because indeed with their success in registering new players it is clear that this makes them even stronger with the presence of some great players there.
As for Atlético they can still do it but sometimes they have to fail in the middle of the road which makes this not very good for them because after they won the title in the 2020/2021 season they seemed to have lost their way last season.
Both Barcelona and Atletico Madrid are the strongest challengers for Real Madrid. Although Atletico often failed because they are decreasing in performance quite a lot in the second half of the season, it is only the history of the previous seasons. History doesn't always repeat, so I believe that Atletico Madrid has a chance to compete for the title. While Barcelona, they are always a strong candidate to win the title each season. Moreover, their squad looks better this season. It is clear that Barcelona has a big motivation to win La Liga title, we can see that they are very active to buy new players.