The Reach = France
This is the most certain, the wine, rich agriculture, the foppish aristocracy stereotype and the location north of the Stormlands and Dorne.
The Stormlands and Dorne = Medieval Iberia + Morocco
The Christian kingdoms and Andalusia were constantly fighting just like the Stormlands and Dorne used to, the Christian kingdoms huddled around the Cantabrian mountains and Pyrenees like the mountainous terrain in the Stormlands, the rainy weather in the Stormlands seems to be based on the autumn and winter storms of the coast of death in Galicia. Unfortunately I have to disagree strongly on the Stormlands.
https://en.wikipedia.org/wiki/Costa_da_Morte
Riverlands ≈ Rhineland
The twins and freys are an allusion to robber barons so I assume the riverlands are akin to Rhineland (though it forks into 3 large rivers). It is viewed as a borderland, the Rhine was used as a natural border by the Romans and surrounding area was often an area of conflict between Germany and France.
https://en.wikipedia.org/wiki/Electoral_Palatinate
The Vale ≈ Switzerland/Wales
lol mountains
Westerlands ≈ Lancaster, England
England isn't very mountainous though the Lannisters are an allusion to the Lancastrians in the war of the roses and surrounding Lancaster are the pennines. I think GRRM wanted them to have a lot of gold which means mines and mountains and that might have something to do with it. The Lancastrians act like stereotypical posh southern Englanders so I imagine GRRM transplanted the Pennines to the south.
Iron Islands ≈≈ shetlands + iceland + orkneyjar
Real vikings had Norway, Sweden and Denmark and these islands were just small settlements and waystations, but I think it was GRRM's intention that they were basically westeros's vikings
The North ≈≈≈≈≈≈ Scotland or Northumbria Northern England or Fennoscandia or Russia
Scotland is north of Hadrians wall so the wildlings might have been intended to be the Scots, The North might just be northern England or the saxon kingdom of Northumbria (maybe where he got the name or the house of Umber). The Starks are an allusion to the house of York in the war of the Roses, but York is not much farther north than Lancaster, though I suppose this doesn't really matter. Geographically the North is huge, but it is cold and has low productivity per square mile, it isn't characterized by mountainous terrain so it is more like Russia or Fennoscandia minus Norway