"This is the first time I have enjoyed a roller coaster" should be used if you mean any roller coaster whatsoever. In other words, use this if you have tried various roller coasters. "This is the first time I have enjoyed the roller coaster" should be used if you are referring to one specific roller coaster, and the listener can determine from context which roller coaster you mean, as, for example, if earlier in the conversation you have been talking about that specific roller coaster.
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