I know they had other types but in Europe if one type was synonomous with one operator Scandi DC9/MD80s must come close. I lived in Stockholm in the 90s and the endless procession of different length DC/MDs was even more striking than BEA viscounts at LHR in the 1960s.
Bit like the Cessna or Piper wings joke that theyw ere all the same but just sawed off the right length for the different models, the MD90 must have come close to being twice the length of the 9-15 .
As for one model companies Air India with just 747s in the 70s-80s?
Linjeflug Now defunct Swedish domestic only flew F28s for a while in the 80s/90s
NLM with just Fokker 50s?