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.
For 1, 2 and 3 I agree with you but I will put Barcelona in 4th position and Arsenal 5th position while Inter Milan is in 6th position, Barcelona performance is not very impressive this season but they can be relied on in important tournaments like this or even Barcelona can reach the semifinals while the final will most likely be between Real Madrid vs Manchester City. By the way, where do you place PSG? they managed to get through a difficult group even though the results are not perfect, PSG still needs to be watched out for, because usually PSG often reaches the semifinals moreover, Mbappe is still there.
I'd agree to most of the ranking but I'd put PSG at least ahead of Barcelona if not Arsenal. Barcelona is quite a disaster this season.
More than the position, the group of 16 is where we heading which would be a two legged knockout. Here 8 of those 16 teams would be eliminated while rest 8 would reach the quaterfinals. Real Madrid, Manchester City, Bayren Munich, Arsenal, Barcelona, PSG, Dortmund and Inter might have better chances to qualify for the next round while the clashes between Inter and Atletico Madrid, Barcelona and Napoli, Arsenal and Porto and PSV and Dortmund could be a competitive one.