That is the surprising part of the Champions League this season. The underdogs play very strong in their domestic leagues (Napoli and Benfica) and the big favorites are all not ranked first in their leagues and also have some struggle once in a while. This is the chance for some new team to finally become the Champions League winner this year. Usually you wouldn't say that Napoli or even Benfica have a chance to win it, but this is year it is a different situation. More and more I lean towards Napoli being the real favorite here. Usually I would have said Manchester City, but Napoli is working like a perfect engine. They are probably the opponent who is most uncomfortable for any team to play against. You just know their results and even though they don't have the biggest name in the soccer world, every other team now knows that they are extremely hard to beat.
I know most persons will love to experience an unexpected team win the Champions league but I doubt that because despite the top favourites having one or two poor performances here and there they seem to move their focus to the champions league and play better in the UCL. I think in the end one of the top favourites will still come out victorious.
But I think that the term "unexpected" is also a question of timing when you declare a team an unexpected or expected winner. Before this season started, not many would have thought that Napoli at this point in time would be among the top favorites for the Champions League title. That is why Napoli would still be an unexpected to winner to me, looking at it from a pre season standpoint. And considering their whole season and performances (actually no luck, just skill), they would be a well deserved winner in the end if they are able to keep their level until the very end.