It's definitely difficult to assess the territories in a vaccum. Yes, supply center territoriess are important but depending on the situations of the game certain non-supply center spaces are worth far more than multiple supply center spaces. Of these North Sea, Ruhr, Tyrolia, Gascony, and Livonia are clear strongpoints that often dictate the game. Yet depending on the dynamics of the game, alliances structures, and so on, these centers could be meaningingless.
More effective stategy is not only contry specific, but situationally so. That's the beauty of Diplomacy and where it takes the principles of chess and randomizes the possible scenarios. There's no way to say what the best strategy is though. It all depends on the other players on the baord, and how you interact with them. Many people insist on a Lepanto opening for Italy, and yet it's incredibly weak and transparrent unless all the diplomatic work has been done in advance, and that you have achieved the desired relationships necessary to make a Lepanto effective. Likewise many people favor the Churchil opening for England, but if France stabs in S'01 you've pissed off Russia, and Germany may smeell blood in the North Sea. Some players buner down as Austria, and other love the Hedgehog. Once again every decision is dependent on what the other six players are doing.
All seven nations are essentially equal at the start. They all have strengths and weaknesses, and the secret to performing well with them is to understand your opportunities and exposures, and compare them with the evolving diplomacy. There is no such thing as 'Oh crap, I drew Italy! I'm screwed!' in Diplomacy. This is simply the mindset of a loser.
Finally I find it rather laughable that any players in this community would be ranking themsleves as a 10 out of 10. That's brass...