They are going to be unstoppable, only Manchester City is capable of stopping Real Madrid from winning the Champions league again, they already had a great team that challenges for every trophy, the same team that won the CL and the LaLiga last season, yet they have added Mbappe to their team. The Frenchman is one of the best players in the world, now adding him into an already world class team is scary for opposition, and at this time, Real Madrid is favorites for all trophies.
Why you only mention Man City? Do you think other teams to have no chance against Real Madrid?
Inter Milan, Dortmund, Barcelona, PSG, Leverkusen, and Arsenal are also strong teams. These teams are not impossible to beat Real Madrid. Although Real Madrid has Mbappe, it doesn't automatically bring the trophy to Real Madrid. Mbappe may need time to adapt with the new teammates and the tactics applied by Ancelotti. Even if we can't deny that Real Madrid is very superior in UCL, we don't underestimate other teams. I also favor Real Madrid, but I don't think only Man City can compete with Real Madrid.
For sure Manchester City shouldn't be regarded as the main club that has the best performance of wining the champions League this upcoming season because there are also more better clubs, however I don't think Manchester City have what it takes to stop the real Madrid because they disappointed me this season when I was thinking that they have performance to win but at the end there strength was not good enough so perhaps if they come back this upcoming season with the same performance they will not have a chance on the champions League, however among all the clubs you listed above I think Borussia Dortmund should not be included because I don't think they will still maintain the performance they had on this season because I can see how they were struggling on there friendly match and losing to a small club.