In my own list this is how I rank em up;
1) Real Madrid
2) Manchester City
3) Bayern Munich
4) Inter Milan
5) Arsenal
6) Barcelona
I do think that Porto would be tough on Arsenal but they'll qualify ahead of them to the next round. I remember the last club that knocked them out of the Europa League Competition last season was Sporting Lisbon. As for Barcelona, they're one team with lot of inconsistency. Could they defeat Napoli? They could finally do that but they will not escape if they fail to sign up a good defensive midfielder. According to Cristiano Ronaldo, Manchester City are his number 1 option to win the Champions League Competition.
The impressive performance of Real Madrid deserves to go with the trophy this season, but what many are thinking will be the barrier to their victory is Manchester City, but to me, I can say that Real Madrid will surely win the trophy, although all how you rank those teams up is looking more like a sure prediction because it might work that way, and Barcelona deserves their position. You put them as the 6th because of their poor performance this season. I see no reason that Barcelona will even think to reach the semifinals because their performance is not up to the standard, but we can’t just conclude.
Inter Milan and Arsenal performances are all the same, but if you compare with effective consideration, you will see that Arsenal performance is somehow more than Inter Milan, but I believe more in Inter Milan than Arsenal. The positive you kept Bayern Munich as the 3rd is also ok because they are also trying their best to maintain the standard, but they should never think of winning the trophy, which is very heavy for the team.