The real answer, which I always give first, is that there is no one time. And that the park is always beautiful and is an ever changing tapestry of sights, sounds, and scents. The natural world is a cycle not a single point in time - this is true even for the manicured, man-created ornamental gardens. Every day of every year has its own unique beauty and unique sights that will NEVER be exactly the same ever again. That too is nature.
"Always beautiful" always falls flat as an answer. The pause of dead-air on the phone or the blank stare if you are speaking in person tells you the listener is quite disappointed in the honest and true answer you have just given. An invisible and silent sigh crosses through your mind and you begin to tell about some of the sights to look for at different parts of the season. Even though you did not answer the question, they are satisfied with this information and move on. And that is so very sad because if they come only at those times, they are missing so much.
My real suggestion as to when to come is this: visit whenever you want and visit often. You will see much and start to learn the character of the place and find that beauty can be found here every day. You will also develop your own list of favorite times and sights which will certainly vary from mine.
