Doesnt the fact that we're arguing about whether Chelsea are best in the world make them arguably the best in the world?
I'd personally put them 2nd behind Barcelona. I'd have probably put Juventus ahead of them as well, but I hear they may have had a bad summer
Milan are challengers too, Bayern have probably lost too much over the summer and rely too much on young players to fill the gaps (Werder for the BL this year), Real Madrid are too disjointed though may be improving. Lyon are good but a step below.
Outside Europe, Boca are winning the Argentinian league but will lose all their players to Italy or Spain before they have a real claim, Corinthians have a good selection of players but are bottom of the Brazilian league, so obviously something is going wrong.
Outside Europe Sao Paulo have the best claim, they won the Copa Libertadores last season, they're in the final again this year, and they're currently top of the Brazilian League, though it is early.