It depends on how big your main base is going to be, especially how tall and materials you are going to use. If very tall stone, you are going to need a lot of iron, so near to a large swamp. Blackmetal is useful for chests, though it is not the most dense storage system, some tin and copper is useful. More copper is required if decorating with it, strong defences require blackmetal and iron.

Wood, stone and blackmarble are all portable so are no issue.

So decide on a metal you need most of and place closest to that.

Near water? Only if you need a port, which is decorative but a pain if you want to keep base secure.

I found a spot on a small area of plains, that had a hill and a stonehenge, surrounded by masses of iron, and within a couple minutes run of copper. Blackmetal is a 10 minute run and tin is harder to get but that is the compromise. Silver I barely use.