Manchester City is the defending champion they will want to retain the title again, Real Madrid could not win the Champions League trophy last season they will definitely try their best to win the Champions League trophy this season. On the other hand, Bayern Munich have made several changes to their team in order to win the Champions League, they will not let any team down.
Mancity and Real are names that are mentioned a lot for this year's UCL championship, but I personally think that the team that needs the most attention this year is Bayern Munich. They are in extremely stable form this season, their players are in extremely good form, especially Harry Kane - a player who is a terror to defenders. It can be seen that Bayern's playing style is less dependent on a specific individual like Mancity or Real. Their midfield is also formidable with Kimmich and rising young star Musiala, giving their midfield depth and good support for the attack above.
In my opinion, Barcelona still has a good squad, it's just that their performance has decreased a little, but this doesn't cause a drastic decline because Barcelona can still fight in the top 4 of La Liga. The defeat against Girona was a little painful, but Girona is on fire so the defeat can still still acceptable this can be corrected by winning other matches and that is what Barcelona has done so far.
Overall, Barca is not weak, but the important factor to end the match is the attack. They are lacking a really sharp striker. They can play a dominant style of play well, holding the ball a lot thanks to a pretty good midfield, but if they don't score, they can't finish the match. Therefore, I think it is difficult for Barca to go far in the upcoming UCL.