Starting mini base goes close to Eikthyr and the Forsaken Stones in the meadows. After you kill Bambi's dad, you can come back to that base later and put up a portal. (If it's your first run, maybe don't put it too close to Eikthyr. If you know how to handle tutorial boss, you can put it as close as you want.)

Second base usually goes in meadows next to the Black Forest and near water for tin and for sailing. Some scouting is needed because you want both crypts and copper nearby. POI farms usually show up in meadows close to BF and can be converted into a decent base. Once you have portals and the karve you can open the map and start looking for your real base location.

My personal preference is in meadows with as many other biomes nearby as I can get. But truthfully once you have portals that doesn't matter a whole lot. Pretty scenery, and water access if you don't mod your portals, is probably the most important thing. Mountain rarely has easy water access. And swamp is too gloomy for me to enjoy spending time there. And by the time you feel comfortable in the plains, you've almost got everything anyway. Black forest isn't bad, but you don't have a lot of flat surface so I'd recommend pine tree houses or towers. Just make sure your base trees are protected from troll attacks.