Favourite to base in is the dark forest, specifically one with an elder boss summon. Dig out underneath, turn it into a bunker, raise the earth back around, get in and out through portals - there's a way of getting metal through the floor by standing close enough.

Ward it - result is a completely impervious and impenetrable base. I also build a portal up to the elder summon itself so when I'm being attacked by, say, an army of fuling's I can summon the elder to take them out, and then finish him off afterwards.
Very satisfying.

Dark forest often borders useful biomes like Plains and Swamp, making it easier to get those resources.