This section of roof sits atop the main face of the building- The entrance. To the left you see the quidditch pitch, the road curving up to the castle as you face due north, and to your right- as well as directly below and against the castle walls- is the lake. Behind you, obviously, the rest of the school.
From left to right, the first area you stand atop would be the Western tower, most well known as the Owelry's large coned head. This is the perfect place to perch, giving one a perfect view of any spot of the castle grounds. Especially one of the greatest places to watch the sun rise or set, apart from designated Perfect-Place-To-Watch-The-Sun-Rise and Perfect-Place-To-Watch-The-Sun-Set regions.
The Great Hall is the next important place on the list. However, due to the enchanted ceiling, going up here anytime before 11pm or after 5 am would be extremely foolish and not at all worth the trouble you'd be in.
After this is the giant cone rooftop that is the Entrance Hall. Have fun finding an easy place to sit, unless you know a pretty bloody good spell on roof-scaling.
After this, a few turrets are lines up all in a row, making a sort of up-down structure to try to go across. Behind this layer of turrets is another layer, a few stories up is more of a saddleback style, perfect for laying on without being detected. This is over an average hallway and classrooms, the roofs for those being a bit more steep.
((Discription deducted, but slightly altered, from information provided by
Hp-lexicon))