For years, I have always arrived at a theme park about half an hour before the park opens, giving me time to get through the front gate and head to my first ride of the day. However, many people have different times they like to get to the park.
Personally, I find that getting to a park slightly prior to opening is the best route to go. Depending on the day, getting to a park at opening will typically be less crowded than say getting to the park in the early afternoon.
Ultimately, it comes down to any constraints prior to getting to the park, and whether or not you mind crowds. Pending on the time of week however, lines for parking and the front gate can become sizable around 1 or 2 o’clock. I can remember waiting an hour for both parking and the front gate on multiple occasions before, and it was not the best thing.
But, in contrast, sometimes the morning can get just as busy. So, like most things I talk about here, crowds can play a big factor.