Red
Red is romantic, because red is the color of love and passion. Consider roses. Red roses mean, "I love you." Orange roses mean, "Let's just be friends," which is synonymous with, "You are dirty, and I hate you." So you do not want to be wrong. Get her red roses, red mattress pads, red irons, red floppy disks, and red aardvarks, and she'll fall hopelessly under your spell.
The Most Intrinsically Romantic Thing Ever
Based on the data above, the single most romantic thing in the universe can be calculated scientifically. It is, simply, a lovable red ear of corn made out of chocolate and shaped like an aardvark holding a trumpet with chipmunks all over it that rescues a wine glass when you throw it.