Croupiest is quite an uncommon word, and therefore finding synonyms for it can be a challenge. One possible alternative could be "most crowded," which refers to a place that is packed with people or objects. Another synonym could be "most congested," which refers to a location that is experiencing heavy traffic or a build-up of something, such as congestion in the lungs. "Most stifling" could also be used, which means a place that is hot and uncomfortable, making it difficult to breathe. Finally, "most cramped" is another synonym, which refers to a space that is small and uncomfortable due to an overabundance of people or objects.