The Citadel from the Half-Life series is one of the tallest buildings in a video game. Standing nearly 2.5 kilometers high, it's hard to even fathom how massive it would be if it were a structure in real life. Before you strain your brain trying to figure out how it would tower over everything, look at this image from reddit. It shows the Citadel in comparison to incredibly tall buildings from around the world. Too see the full picture, keep reading.
Of course Half-Life's Citadel has nothing on Mass Effect's Citadel.