Your name is
I am a triumph, a thundering wall;
Refers to victory (triumph), and to the Victoria Falls in southern Africa, aka "The Smoke that Thunders"
a goddess, a ruler the grandest of all.
Victoria is the ancient Roman goddess of Victory, Queen Victoria of the UK could be considered one of the grandest rulers in history, certainly the British Empire was huge during her reign
I loom over towers that can touch the skies;
Could refer to Victoria Peak, the highest point on the island of Hong Kong (where there is a large city with many towers), otherwise there are many Mount Victorias in the world
few of world's waters can rival my size.
Refers to Lake Victoria, second largest freshwater lake in the world, biggest in Africa
A landmass colossal, a forebear of kings;
"Landmass colossal" might refer again to the old British Empire, could also refer to Victoria Land in Antarctica or the state of Victoria in Australia.
Forebear (aka ancestor) of kings probably refers again to Queen Victoria, who was succeeded by several kings, up until Queen Elizabeth
on seven continents my name still rings.
Due to the extent of the old British Empire, you can find cities, provinces, states, lakes, mountains, everything named after Queen Victoria on every continent in the world
Edit: Due to the OP's comment, I think "A landmass colossal" may actually refer to
Victoria Island, 2nd largest island in Canada and 8th largest in the world