Everyone will have their opinion about best and worst coasters, but truly, they all have their great points, depending on what you're looking for. Airtime, a softer ride, speed, quick turns, etc... I agree with Soggy, ride them all, as best you can!
I'd suggest hitting a big one just as the park opens: either Raptor or Millenium Force, because that way you can wait a shorter amount of time for them, probably. Take on small ones right away afterwards, because most everyone will be running to the big rides and you'll be able to get right on the smaller ones without more than a 15-20 minute wait once they open. Then, when the lines get really long in the afternoon, you can hit non-coaster rides, shows, eating (after which smaller coasters: Disaster Transport, Mine Ride - are still okay), games, etc., or go ahead and wait for a couple more of the big ones - Mantis, Mean Streak, etc, since the lines for everything will be longer anyway. I'd suggest leaving the Mean Streak and Gemini until nearly closing, because most people tend to drift toward the main gate and those lines really shorten up.
When you go will definately have an effect: avoid August and July if you can, because the lines are much longer, not to mention the hotter weather then. My recommendation is for mid-June up until about the 24-25, since everything will be staffed, stocked, and open, but the lines are still shorter and the weather cooler, and school trips are tapering off.
But definately try to get to all of them, and don't forget the other great things at CP too! :)
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PO!NT OF VIEW - A Different Look at Roller Coasters
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