What is the first western European nations that competed for world dominance through exploration?

I'm not great with my european history from that time period, but I would guess the brittish would be the first as the roman empire began to retreat. Spain, France and Portugal, the other early major western European colonial powers, are closer to rome and I believe formed their empires later. It wouldn't surprise me if Portugal was first, though, as they are on the far side of spain and fairly well insulated from the roman empire. Sorry I can't be more sure.