Thanks mongoose998. You've started a very interesting thread and I think you should be glad of the interesting replies you got. I could agree with most of what all the guys said, just noticing that the importance of a HSC (as well as the importance of any other territory) changes when you're expanding rather than when you're defending. Or at the beginning of a game rather than middle rather than ending. But I'd like to look at the question from a different, aritmethical, point of view.
I wondered a question similar to yours, some time ago. Which are the most important territories? So I started building different tables, related to which criteria I deemed the most important each time. The simplest table, assigns a score to each territory, this way:
1) Each bordering territory = +1
2) Each bordering SC (including itself) = +1
So Portugal got 2 Bordering Territories (MAO & Spa) and 2 BSCs (Spa & Por itself): total 4.
This system has some obvious flaws. It doesn't count stalemate lines, first of all. I.E., it looks weird to find StP under average (avg=8.32).
But I believe that this simple table gives to you a good index of the importance of each territory, regarding the possibilities that owning it can give to the owner. This is the table:
North Sea 17
Serbia 12
Kiel 12
Rumania 12
Ionian Sea 12
Mid-Atlantic Ocean 12
Trieste 11
Black Sea 11
Baltic Sea 11
Burgundy 11
Galicia 11
English Channel 11
Budapest 10
Bulgaria 10
Munich 10
Norway 10
Venice 10
Aegean Sea 10
Gascony 10
Tyrolia 10
Tyrrhenian Sea 10
Constantinople 9
Moscow 9
Denmark 9
Smyrna 9
Sweden 9
Ruhr 9
Ukraine 9
Livonia 9
Silesia 9
Ankara 8
Berlin 8
Greece 8
Holland 8
Prussia 8
Rome 8
Sevastopol 8
Spain 8
St. Petersburg 8
Vienna 8
Belgium 8
Liverpool 8
Warsaw 8
Albania 8
Apulia 8
Armenia 8
Picardy 8
Piedmont 8
Gulf of Lyons 8
Norwegian Sea 8
Western Mediterrane 8
Brest 7
Edinburgh 7
Finland 7
Heligoland Bight 7
Skagerrack 7
Adriatic Sea 7
Bohemia 7
Gulf of Bothnia 7
Tuscany 7
Wales 7
Naples 6
Paris 6
Marseilles 6
Irish Sea 6
North Atlantic Ocean 6
Yorkshire 6
London 5
Tunis 5
Clyde 5
Eastern Mediterranean 5
Portugal 4
Barents Sea 4
North Africa 4
Syria 4
So, from this point of view that may look rather abstract (although the question was rather abstract, too), my answers are:
Germany - Kiel
Austria - Trieste
Italy - Venice (Just Because I Can't Say Ionian Sea)
Turkey - Constantinople (Just Because I Can't Say Bulgaria or Black Sea)
Russia - Moscow
England - Liverpool (JBICS North Sea)
France - Brest (JBICS Burgundy)