I personally can't put City as favorites to win back 2 back UCL as this is something that rarely happens in this competition. City has a great team but also fights for EPL title which might be an important factor in UCL and Real Madrid is the best team that knows how to speculate games like this and won't be surprised to see Real qualifying further but on the same time I can't see them winning the trophy either.
I will stick with my theory that whoever eliminated Porto in the last years , ended up winning UCL so this year , Arsenal should be one.

Anything can happen, we can see how Real Madrid with 3x UCL trophies in a row then the possibility of Manchester City will fight for the UCL again but certainly we cannot determine that exactly because in the UCL competition it is still difficult to guess even Real Madrid or other teams.
Is it true about that theory? Whoever eliminates Porto they can win the UCL? I've only heard this theory because I honestly didn't notice when the winning team beat Porto first.
Who knows, Arsenal is not the favorite can also happen this season with the title with Arteta.