Wow, where does one start? The place is stuffed with monuments, big as it is. Depends whether you want intrepid (ie rambling over miles of moorland) or convenient (close to a road). I wouldn't miss the following:-

White Moor (serious trek)
Greywethers
Scorhill
Yellowmead
Merrivale Stone Rows/circle
Lakehead Hill
Down Tor
Brisworthy/Ringmoor

I reckon that's enough depending on how long you've got and how far you want to walk. I'm sure others will have lots more. Have fun!