well..'best season' is open to different interpretations..and I understand fully where Palos and everyone else is coming from. There is no correct answer...But like kicker said, big teams count trophies and consider all else failures.
I still believe that winning only the Carling Cup is not a good season for Chelsea no matter how many finals and semi-finals they make..no matter how many obstacles they overcome. For me, I am considering what a team expects of itself in the equation and I think every Chelsea player, Mourinho and Abramovic would consider the season a failure if that is all they won. Chelsea can salvage their season by winning the league and/or FA Cup. But if ManU win those two...they would have had the better season. You would be hard pressed to get the players, managers and the club owners to disagree. But we have to wait and see who wins what.
So far Sevilla is having the best season...they are still in the running to win EVERYTHING they could possibly win. There is no getting better than that. No other team in Europe can boast that. Yet, Sevilla still has won nothing...so again we have to wait and see. But because they are a small team, they will gain more pride and joy from a close call...2nd place in La Liga for Sevilla will be greeted with far more joy than 2nd place in the EPL for Chelsea (IF that is how things end). Winning the UEFA Cup will be clebrated in the streets...Chelsea, however is bitterly disappointed at losing at the SF stage of the CL. Doesn't matter if it may have been harder to get where Chelsea got because they consider a failure
Somewhere it is a mixture of what you win , plus what you achieved given your resources and expectations. The first part has not yet been figured out..so we'll have to wait and see. Winning titles is still the most important part of the formula for big teams