Picking a park as favorite or nicest is like picking the favorite or nicest woman...they're all beautiful.
But, if I had to pick one, I like the beauty of Bandon Beach, Oregon. It's unique Geology is of the oldest in the world. It's on a fault line similar to that of Cornwall, England, if you've ever been there. Time has eroded the rock to form my favorite image in the world.
Here's a photo: (I posted some other places I like on an album of "Nature Photos" in my profile with locations in the comments.) These places include:
Crater Lake National Park
Smith Rock State Park
Columbia Gorge Scenic Area
Multnomah Falls Scenic Area
Tillamook Head State Park
East Willamette Valley
Gifford Pinchot National Forest
Agate Beach State Park
Yaquina Head Lighthouse at Agate Beach
Steamboat Springs, Colorado
Hundreds of places just locally, and tens of thousands more all around.
My favorite beach to walk on is a 5-mile stretch of sand at Manzanita. Bandon Beach is an unpleasant walk for most people, with high winds that blow sand in your eyes and constant drizzle, but it has it's days, and for me, the scenery far outpaces any harsh weather conditions. I don't like heat, anyway, so it's fine by me. Give me cold and wet over heat anyday.