The cupola in Philbrook Gardens.  I proposed to Linda in 2005at that location. Brought a small boombox and had our favorite song (Wonderful World, Louis Armstrong version) ready to play.  Got her up there under the guise of saying we would have our picture taken by someone wandering around.  Pulled out the music and pressed play, got down on my knee and asked her. It was great.  And got our photo taken by someone passing by, just like I said!
It's all about the set up, not just the location. Have a place to go to dinner or lunch, a NICE place, afterwards.  Have a camera!