Which Nations Are Best for Turtling?

[Illuminated One]

Well, somehow along AoE's lines (if I understand him correctly) - turtling to invest in defense is not what you want to do.
You want to invest as much as you can into thing's with a return (research, gold, gems). Once you get that return you use it to overrun underdeveloped enemies.
Defense, i.E. military forces, has no return only an upkeep, so you have to avoid it as much as you can, if you are following that strategy.

The problem is that dominions mechanics are so unsuited to this.
Gold is proportional to the provinces you have, since there is no need to somehow establish order (compare Cyrus the Great's strategy to Alexander the Great's - Alexander would have won in Dom but his empire held how long, until he was sick?)/develop it (forts pay off in 40 turns, a new province pays of in 1 turn).
What's more gold is most important early on, so you're only going for RoI.

Gems - well, clams and co. which are admitedly flawed.

Investing into research = Investing in defense since all the mages around are also your combat mages. This also a reason that a nation that's effectively dead can kill it's conqueror. I have killed almost the entire army of a player that was taking my provinces simply by charging in my researchers, which was enough to end that war it seems, but make a comeback from 3 provinces, no way.

So, don't turtle, always rush and never join a fair war (edit: Well, somewhat offtopic but since several people have remarked on that being a strategic no-brainer, is it really? Rome's Empire was not created by fighting against pushovers, Elisabeth's England picked a superior foe and laid the groundwork for the British Empire), best is a dogpile.
Unless, maybe you can get vampires like Bogarus, which could be good for turtling.

edit: So that's the dirty secret of the bloodhungry Bogarus?