When I look at the matchups I think that the most difficult to predict result can the Napoli v Barcelona and Inter v Atlético Madrid. I mean theory there are favourite of this matches which is Barcelona and Inter. You know Barcelona are having really tough days and Xavi even announced his parting ways with Barcelona this summer. Napoli could make a surprise. Also, Atletico Madrid plays very consistently in Laliga as well as Inter Milan in Serie A, so these two clubs will play carefully, aggressive and defensive, play both of them are equally good.
Barcelona seems to be picking up form as the season progresses. Their biggest problem now is Injuries. So many of their players are out injured. Napoli too seem to be getting better even though they still struggle. I think Barcelona is the favorite for this tie but they have a very little advantage over Napoli.
The more difficult tie is Inter vs Atletico Madrid. Inter Milan is in very good form while Atletico is a very stubborn club that never likes to be beaten by a fellow big team. They love a challenge. They might lose easily to a mid-table team in the league but when it gets to Madrid and Barcelona, they are ready to play their lives out on the pitch to get the win.
I feel like that's how they'll be against Inter in both legs. I honestly do not know who to favor in this tie.