Having lived around Dartmoor for 25 years, I would be a little biased. But my favourite sites are Whitmoor Circle (Little Hound Tor Circle) - a bit of a trek, but one of the best (if not THE best) circle you will ever visit, Scorhill Circle, Grey Wethers double circle (which can be combined with Fernworthy circle and Assycombe Row), Cosdon Stone Rows and Cosdon Beacon. Drizzlecombe rows, Brisworthy Circle, Shovel Down..... god, I could go on, the list is endless- but you would need much more than a week !!!
Then of course as TSC says - there's West Penwith - Boscawen Un, Merry Maidens, Men an Tol, Maen Scryfa, Chun Quoit, Mulfra Quoit, The Pipers of Boleigh, the Blind Fiddler...... Most of the sites in West Penwith are relatively easy to get to.
Oh.... and don't forget Bodmin Moor between Dartmoor and West Cornwall !!!!