Which Nations Are Best for Turtling?

[Quitti]

Depends if you're playing SP or MP. AI usually does not consider scales or dominion effects, but your best bet in SP would probably be a water nation, and make sure that there are no AI water nations in game (or eliminate them).

In MP it's a bit different story - the nations with specific dom effects, such as ma C'tis' miasma keeps people out or at least makes people think twice before invading - same is LA R'lyeh's insanity dominion. Also nations with top-of-the-line PD might work if you invest gold in it (usually not a good idea, since it'll only do so much). Also if you got abysia as a neighbor, and you have a (national preference for) cold3 scale, neither of you really want to step into each others lands.
Also, power projection always keeps people at bay, that is, stockpile huge amounts of troops into somewhat non-threatening places where they are still seen well enough. It might deter the enemy enough to not attack you.