There is really no team at the moment that is playing a perfect season. Real Madrid is strong, Manchester City isn't playing perfectly, Bayern Munich had some problems during the group stage in some games and there is no team in my opinion that looks so dominant at the moment that it has a unique favorite role in the Champions League. Arsenal has had some bad results despite playing quite well, but that doesn't mean a lot for the Champions League I believe. But Porto should be an opponent that Arsenal can handle. The coming weeks are important for them and there is also the FA Cup game against Liverpool next week.
If we talk about the perfect club we will never find one like that, maybe they never lose but they still draw. So far I've noted Leverkusen who can do that but they also still draw, yes even though they never lose. In football a draw will still count as an unsatisfactory result.
All with their own advantages and disadvantages, every time 2 clubs meet, they will always look for gaps where their opponent's weaknesses lie and if they can be utilized properly they will be able to win the match.
But other factors also sometimes affect the outcome of matches, one prominent example is luck. No matter how strong and how many chances a club can create, they won't be able to score if they are unlucky in the match.