Been saying for a while that France was the most defensible power! Turkey's defensibility is overrated, big time; I remember once last summer that Scmoo472 and I happened to draw Russia and Austria three times in a row in consecutive live games, and each time we had a different Turk who never really talked much, so both being fairly chatty players, we quickly ended up deciding to smash Turkey all three times. By the third time we'd perfected the art of KOing Turkey by 1903 unassisted by Italy. The thing about Turkey is that if it can keep Russia out of the Black Sea, it becomes infinitely more defensible... absent BLS, you need the Lepanto to work (which is no guarantee, as it's easily blocked) or else you're faced with slogging it out until 1904-05+. But if Russia manages to score the Black Sea in S02 (we pulled this off all three times), Turkey's position is broken, and it's just a matter of time.
I would say England is about as equally defensible, but not in the same way. Where Turkey is predicated on keeping hostiles out of 1 or maybe 2 specific places on the board, England is predicated on your ability to avoid an early convoy to the home island. Once that army goes it's basically all over - it's rare for England to keep its army at home, so your army has a massive advantage in horizontal movement (able to jump immediately from Yorkshire to Liverpool, and support moves to Edinburgh from Liverpool), and England, like everyone else, has some very weak non-SC spots (Yrk, which covers all 3 SCs, and both Cly and Wal which touch 2). England is probably more defensible than Turkey in that while invading is easier due to having five easily-accessible sea provinces to use as launching sites for the invasion, actually killing it once the marauders land is much more difficult.
And then there's France. I've never seen France die before 1905. I've managed effectively to kill it by 1903, but both times were as Germany with a willing Italian ally, and involved breaking both Burgundy and Piedmont DMZs on the first turn; I challenge you to find a country that could survive 2 hostile neighbors breaking DMZs right off the bat! France has the frontal access of Turkey with the internal defensive cohesiveness of England, and an extra SC, to boot.