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John H.Is a box of '52 Topps high numbers too obvious an answer? If so, I would have to go with any Topps or Bowman box that gives me a chance to pull a Teddy Ballgame card. Next would be a first series 1964 Topps box. I was only five at the time but I know I had some of them, including a Willie Kirkland (card #17) that outlasted all the rest.
Edit: I forgot all about the '59 Fleer Ted Williams set, probably because I already have it. I would love to open a box of those hoping for a mint set, including #68.