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.
Yes, winning Champions League back to back is not an easy task, it requires experience and quality players. Manchester City is a great team no doubt, but winning the UCL back to back will be difficult for them because they will be playing against the best team in the UEFA Champions League history in the quarter final, and even though they were able to eliminate Real Madrid, I don’t think they can win the Champions League this season because there are other teams that are hungry for Champions League trophy and they will use this opportunity to see if they can make it.Quote
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. 
I will be very happy if the history repeats itself. Arsenal this season have the quality to give it a try; eliminating Bayern Munich will be the main challenge for Arsenal which if there were able to do so, there is no doubt Arsenal can win the Champions League this season.