The Colosseum is, in my opinion, one of the most overrated roman sites of all times. It's terrible crowded, restored into ruins and there's frankly enough, not very much to see. I'm not even sure that I would add it to a list such as this one.
The Vatican museum would be placed a little further down. Yes, they have some of the most impressive pieces in the world, but the museum itself makes it painful seeing them.
More later, when my paper is finished.