I’ve found that synesthetes usually have arbitrary favorites: “I like 4. I don’t know why. I just do”. Nonsynesthetes almost always answer in one of three ways: “No, I’ve never thought about”, “Yes, because it’s a significant date*”, or “7”.
My favorite number is 9 (followed by 7 and 3). I personally cannot fathom how someone could have no preferences whatsoever- I see all letters and numbers as being so different. But that’s just the way my brain’s wired.
*Or significant anything, really. Dates, addresses, SS#- whatever it is, they picked the numbers because of their associations.
I like 7 and 10. 4 and 21 as well, but only if they’re together. 7 just looks nice, and 10 seems stronger than all of the single-digit numbers. 4 and 21 because something very nice happened to me on April 21st, and now the numbers have a new connotation in my mind.
Also 00. It’s not really a number AFAIK but they put it on the roulette wheel anyway. I chose it for the back of my softball jersey because then when I ran out on the field, people could point at it and say “00!”