Meadows with good boat access, and must be close to the Black Forest (helps with early ramp up) and having a beech forest nearby for a endless supply of wood. I typically also build a fortification in the plains late game, and plenty of small "safe houses" in various locales, usually next to swamps, mountains, etc.

I frequently raze Draugr Villages for my meadows base, as the pre-flattening is nice.