Is there an online engine like Google Scholar which allows me to sort the papers by citation count?
There are at least four freely available databases which have this function. In random order:

Lens: In Lens, instead of the default Sort by Relevance choose Scholarly Citations (highest)
Microsoft Academic: In Microsoft Academic, choose SORT BY MOST EST. CITATIONS
Dimensions: In Dimensions, choose Sort by: Citations
ScienceOpen: In ScienceOpen, select Sort by cited by count