While now, we are heading for the quarterfinals of this season now I am feeling all teams will be gone with their full strength and never try to underestimate their opponent because from here any mistake of miss judgement can bring them down which could be worse currently both teams are at the same level even Manchester City is looking stronger on papers due to their strength, but Real Madrid is also giving his best and despite having few injuries they are giving their best in all competitions which is impressive.
Even nothing is predictable and there is no guaranteed about any team's win in final as well, but this is going to be one of the best encounter in this season mostly fans are expecting Manchester City is favourite and also able to win, but things could be not easy for any team as this is wide open game for the both sides.
You are right in times like this is very unwise for a club to underestimate any team because anything is possible and for any club to qualify to this extent has already shown that there is a good performance in them, so this is the moment were every club needs to tighten there pattern of play because like you said any slightest mistake could result to losing of match, however in terms of knowing the exact club that has the best performance to win may likely be difficult to identify because sometimes a club we feel may not go far could sometimes surprises everybody, although at first I had Manchester City, Real Madrid and Bayern Munich in mind that will win the match because majority of people also predicted it but for now nothing is certain because the matches could turn around but however the only time we can really no the club that could possibly become the champions League winner is the clubs that would qualify from the quarterfinals.