The UCL is the league to know the best in Europe but at times, it is not actually the yardstick to know the best, situation can happen, which is not limited to luck and lack of luck on the part of the less-deserving team and more deserving team respectively. Averagely overall, the UCL is a good league to know some of the best and of course, those reigning in a certain league might be messed up when another better league champion shows their supremacy. Some local leagues are generally weak but will not soon know until they play tourneys like the UCL since they are competing with the likes of weaker team locally.
Yes mostly people think UCL is best competition to find out which teams are best in Europe. But it is not always perfect way to measure how good team is because many factors affect on team like luck or injuries or how well individual players perform. By the way UCL usually shows some of best teams in Europe it is also true that teams who win their own country league might struggle when they play against stronger teams from other countries. Also teams from weaker leagues in their own country might not realize how good or bad they are until they play in UCL where they have to play against teams that might not be as good in their own country league.