Top three candidates and favorites to win the Champions League has got to be Manchester City defending the Champions League Competition, Real Madrid who have won fourteen times and lastly Bayern Munich who are also hot favorites having a coach who has experience winning the Champions League Competition before with Chelsea.
Paris Saint Germans wouldn't want to lose the big man Kylian Mbappe too early even if he'll leave at the end of the season, they'll want to try again this season but I doubt he'll be sold out.
I don't think the transfer window can change nothing. It's football though and it's always unpredictable.
I certainly agree with you about the favorites to win the UEFA Champions League this season. Real Madrid are looking too good at the moment with Bellingham killing it for them in every match they play. They dominated their league and they were the group topper.
Bayern Munich on the other hand looks too good as well with Harry Kane doing a very impressive job. Manchester City is a bit out of the touch right now due to the injury of some of the key players and they are coming back. I don't think Bayern or city will buy another player this January who could change the result of Ucl. Real Madrid could be the one. Mbappe might join Real this January and as @Distraction said, it will have a massive impact on the result of ucl.
Who is winning and who is going to be toughest its good for the talk because right now few changes are going to happen in teams with 6 weeks left in start of next round, and we will have recovery for few teams with many players are on transfer list as well which will bring strength for few and few will be going in trouble just because of this currently talking about who is going to win it's not easy even currently three teams are tough and can go through with Real Madrid, Bayern Munich and Manchester City, but we can't ignore Inter Milan and Arsenal which are also good and have enough sources to win the title but it's never been easy for any team to go through for the win in ground Manchester City looking for some buying in January and other two teams are having good squads which can do good without any changes so first just wait and watch about who is going to have good in early rounds for the win in this season.