best faction to turtle with?

[Quirinus]

Definitely not Germania, or any barbarian faction in RTW, IMO. I mean, blitzing is a must for barbarian factions, before they are brought under by squalor problems.

The Julii, IMO, is the best faction for turtling, because of its easily defensible frontiers (or the Brutii, who have no hostile frontiers at all!), and the fact that they only get better with time (Marian Reforms).

In BI, maybe the Saxons or the Sassanids. Both can command a "corner" of the map fairly easily. Apart from those 2, the randomness of BI means it's hard to predict how any faction might fare as a turtler.

What about the Celts? You can't beat having an isolated island for turtling. The Berbers are not bad too once you've driven the Romans from former Punic Africa.