Way way back in time, it would be the Mill Race. But that was when trips to CP were rare gems (translation: before I started getting a season pass). Now, it's dependent on a number of factors, such as "Who am I with?", "What time of day am I arriving?", and "Who let all these other people in the park to make all of these lines so long?" I can't think of any single ride that I would not ride first during any given trip to CP, but Magnum seems to edge out the rest of the competition during most trips.
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"Thank you for riding [fill in name of ride], and enjoy the rest of your day here at [fill in name of park]."
CP trips: 07
MF rides: 09