Centennial, Cascade, and Willamette are all good dry-hopping hops... but I'm not sure dry-hopping is what you want to do.
Dry hopping adds hop aroma to your beer. If you just want your beer to have a lot of hop aroma, chuck 'em in, the more, the merrier.
However, if you're looking to counteract sweetness from using more extract than you planned, dry hopping won't help you -- you need to boil those hops to extract the bitterness. Do a forum search for "hop tea."