I think there's no need to discuss about Barcelona vs Napoli and Arsenal vs FC Porto, it's a sure win for both Barcelona and Arsenal, Napoli and FC Porto aren't that good compared to last season.
What I think we need to pay attention is PSG vs Real Sociedad and Inter vs Atletico Madrid, I believe most people will think PSG and Inter will win since both of them were strong in the last season, but I believe this time Real Sociedad and Atletico Madrid able to prove themselves even though they're underdog.
How can you call Athletico Madrid under dog,do you know what they are capable of doing? I even think they can surprise Inter Milan because they are doing an incredible work this season,they are not far behind in the Lalliga table,and with their performances this season,you can't underrate them like that.
I know Milan are ontop of their league table,and with what they did last season has given them the confidence to press more this season, couple with the ability and confidence to progress to the next stage of the competition,but facing Athletico Madrid is really a hard nut to crack,and they must be ready to fight it hard if they are to ever win in this encounter,else, Athletico will surprise them.