It is always difficult to determine who will win the Champions League title, especially when the season has not started. Manchester City certainly have a chance to repeat their success, but to defend the Champions League title then I think it is the most difficult challenge for any coach. Pep Guardiola is expected to keep his chances good, but all the teams that will be competing in the Champions League will also be pushing for the title.
Yep, Barcelona is the biggest contender in this league, Bayern Munich can be a strong contender as long as they can sign Harry Kane.
For Real Madrid, PSG, Inter, Milan, Arsenal etc, they're still in development phase to find their best eleven players.
Surely Manchester City is still look as the strongest club, but they're not yet in the same level like Real Madrid in 2015-2017 seasons where no one can compete them.