Oh gee ... just one marble? A 1" Marble King BumbleBee with a classic patch-and-ribbon pattern.
If I could have two, the second one would be a 1" Akro Superman, with evenly balanced colors in a simple clean corkscrew pattern.
Keeping it simple and classic.
Uh oh ... now I'm remembering my spotted dick and the other marbles in my advent calendar.
Nope, sorry, I just broke my brain.
But seriously, when I go to my nursing home, any single marble will do to capture this part of my life, probably a large size for better viewing, and better if it was a gift.