Good to see another Aussie on the boards.
Last year I flew to Cleveland after I got to LA, did Cedar Point. Then went to New York and from there I did SF New Engand, Hershey, and Great Adventure. Then I went on a tour that stopped at Orlando, but I only had time to do Sea World, Islands of Adventure, and Universal. Then I flew back to LA and did some parks there.
Personally, Cedar Point is the best - loved it. Islands of Adventure was the best in Orlando. Busch Gardens I wish I could have gone to for the coasters, and with the new dive machine looks like a must visit. I have been to the Disney ones on a previous visit and they are worth going to if you have never been before but they are not really for thrillseekers. Great Adventure has some good coasters but it's no Cedar Point.
Washington DC is good place to visit and it has a heap of parks all within two hours - Six Flags America, Paramount King's Dominion, Busch Gardens Williamsburg, Dorney Park, Knoebels, Hershey etc. Unfortunately I didn't have time to do this myself.