Geography: I tend to go for islands or peninsulas that are close to a bigger island so I can build an easily defensible base and a bridge to the big island so I can transfer ores easily. Always on the coast (no reason why you shouldn't build on the coast).

Biomes: Either Meadows or Black Forest. I do tend to make sure there are mountain, swamp and plains biomes nearby (preferably on the same island) so I can gather all late-game resources and bring them to this one base. Otherwise you'll be building many bases and struggling to transfer resources.