Successful may have many definitions. The one I prefer in sport is making the best out of a given situation. My first team won the treble this season but I do not think this was my best achievement. I am likely to win the title with my second team, which is by miles much weaker and plays in a much more complicated league (basically, on paper, they are the 5th or 6th best side). So, I am very proud of my treble but my best success will probably come from my second team.
An example in real life: some people consider Guardiola as the best coach in the world. But he only managed Barça and Bayern ! Who on earth would not win titles with those teams ? Compare with what Ranieri's doing at Leicester ! For me, Leicester is a much more successful team than Bayern.