Not quite sure how to understand this. Do you mean which parts were culturally most Greco-Roman? (Both Greeks and Romans would likely be horrified to hear that Greek counts as Roman).
Northern and Central Italy would have to count as the heartlands of Roman culture, with Southern Italy and Sicily home to a Graeco-Roman cultural melange. Southern Gaul (the Cisalpina and Narbonensis) and Spain were also heavily Romanised and quite populous, while the notherrn ansd mountainous partzs were both less populated and less Roman. The western Balkans (Moesia and Illyricum) were less Romanised. I'm not sure about North Africa, but it looks like Romanisation, though quite thorough, was mostly an urban, coastal affair.
The heartland of Greek culture would be the peninsula, western Anatolia and the islands. In terms of population, Anatolia probably outweighed what we consider Greece proper today. The cities of Syria and Egypt were also Graecised, but existed in a sea of culturally different peoples and took on traits that mainland Greeks would have considered barbaric. The rural areas were less Greek.
Of course, it all depends on what you consider "Roman" or "Greek". I would argue that a place like Londinium or the Colonia Claudia Ara Agrippinensium (Cologne) was culturally Roman, just Roman in its own way, the same way that Harlem or the bayous are American even if they differ from Beacon Hill or Richmond. The same - more so IMO - applies to Alexandria or Antioch being Greek. Especially in the Attic heartland, there was an upper-class opinion that dismissed these lesser, koine-speaking, bastardised Greeks as culturally inferior. They themselves naturally did not share that opinion, and I am tempted to agree with them. Alexandria certainly looks to have been a much more interesting place than Athens, however much you could argue the degree of its Greekness (parts of it certainly looked Egyptian and Judaic).